# The deal.II FAQ

This page collects a few answers to questions that have frequently been asked about deal.II and that we thought are worth recording as they may be useful to others as well.

## General questions on deal.II

### Can I use/implement triangles/tetrahedra in deal.II?

This is truly one of the most frequently asked questions. The short answer is: No, you can't. deal.II's basic data structures are too much tailored to quadrilaterals and hexahedra to make this trivially possible. Implementing other reference cells such as triangles and tetrahedra amounts to re-implementing nearly all grid and DoF classes from scratch, along with the finite element shape functions, mappings, quadratures and a whole host of other things. Making triangles and tetrahedra work would certainly involve having to write several ten thousand lines of code, and to make it usable in all the rest of the library would require auditing a very significant fraction of the 600,000 lines of code that make up deal.II today.

That said, the current specialization on quadrilaterals and hexahedra has two very positive aspects: First, quadrilaterals and hexahedra typically provide a significantly better approximation quality than triangular meshes with the same number of degrees of freedom; you therefore get more accurate solutions for the same amount of work. Secondly, because the shape of cells are known, we can make a lot of things known to the compiler (such as the number of iterations of a loop over all vertices of a cell) which avoid a large number of run-time computations and makes the library as fast as it is. A simple example is that in deal.II we know that a loop over all vertices of a cell has exactly `GeometryInfo<dim>::vertices_per_cell` iterations, a number that is known to the compiler at compile-time. If we allowed both triangles and quadrilaterals, the loop would have `cell->n_vertices()` iterations, but this would in general not be known at compile time and consequently not allow the compiler to optimize on.

If you do need to work with a geometry for which all you have is a triangular or tetrahedral mesh, then you can convert this mesh into one that consists of quadrilaterals and hexahedra using the `tethex` program, see https://github.com/martemyev/tethex .

### I'm stuck!

Further down below on this page (in the debugging section) we list a number of strategies on how to find errors in your program. If your question is how to implement something new for which you don't know where to start, have you taken a look at the set of tutorial programs and checked whether one or the other already has something that's close to what you want?

That said, there will be situations where documentation doesn't help and where you need other someone else's opinion. That's what the deal.II mailing lists are there for: Feel free to ask! You may also wish to subscribe to the users' list -- not so much because someone else might ask the same question you have, but because reading the list gives you background information on things others are working on that may help you when you want to do something similar.

When asking for help on the mailing list, be specific. We frequently get mail of the following kind:

```  I'm trying to do X. This works fine but it fails when I try to transfer
the data to my MyClass::Estimator object. I tried to use something
similar to what's done in a couple of tutorial programs but it doesn't
work. I'm new at C++ and I just can't seem to get the syntax right.
```

This message doesn't contain nearly enough information for anyone to really help you: we don't know what `MyClass::Estimator` is, we don't know how you try to transfer data, we haven't seen your code, and we haven't seen the compiler's error messages. (For more examples of how not to write help requests, see Section 3.2 of this document.) We could poke in the dark, but it would probably be more productive if you gave us a bit more detail explaining what doesn't work: show us the code you implemented, show us the compiler's error message, or be specific in some other way in describing what the problem is!

### I'm not sure the mailing list is the right place to ask ...

Yes, it probably is. Please direct your questions to the mailing list and not to individual developers. There are many reasons:

1. Others might have similar questions in the future and can search the archives.
2. There are many active users on the mailing list that are happy to help. There probably is someone who did something very similar before.
3. Imagine everyone would stop using the mailing list and email us directly. We would spend most of our time answering the same questions over and over.
4. Many users are reading the mailing list and are interested in deal.II in general and are learning by skimming emails. Give them a chance.
5. As a consequence of all this, we typically prioritize questions on mailing lists over emails sent directly to us asking for help.
6. Don't be afraid. There are no stupid questions (only off-topic ones). Everyone started out at some point. Asking the questions in the open helps us improve the library and documentation.

That said, if there is something you can not discuss in the open, feel free to contact us!

### How fast is deal.II?

The answer to this question really depends on your metric. If you had to write, say, a Stokes solver with a particular linear solver, a particular time stepping scheme, on a piecewise polygonal domain, and Q2/Q1 elements, you can write a code that is 20% or 30% faster than what you would get when using deal.II because you know the building blocks, shape functions, mappings, etc. But it'll take you 6 months to do so, and 20,000 lines of code. On the other hand, when using deal.II, you can do it in 2 weeks and 204 lines (that's the number of semicolons in step-22).

In other words, if by "fast" you mean the absolute maximal efficiency in terms of CPU time deal.II is more than likely to lose against a hand-written Fortran77 code. But for most of us, the real question of "fast" also includes the time it takes to get the code running and verified, and in that case deal.II is most likely the fastest library out there simply by virtue of the fact that it is by far the largest and most comprehensive finite element library available as Open Source.

This all, by the way, does not mean that we don't care about speed: We spend a lot of effort profiling the library and working on the hot spots to make codes fast. The discussion of this issue in the introduction of step-22 is a good example. There are also some guidelines below on how to profile your code in the debugging section of this FAQ.

### deal.II programs behave differently in 1d than in 2/3d

In deal.II, you can write programs that look exactly the same in 2d and 3d, but there are cases where 1d is slightly different. That said, this is an area that we have significantly rewritten, and starting with deal.II 7.1, most cases should work in 1d in just the same way as they do in 2d/3d. If you find something that doesn't work, please report it to the mailing list.

Historically, the differences primarily resulted from the fact that in deal.II, we represent vertices differently from lines and quads; whereas the latter can store information (for example boundary indicators, user flags, etc) vertices don't. As a consequence, the boundary indicator of a boundary part in 1d (i.e. either the left or right vertex) were determined by convention, rather than by setting it explicitly: the left boundary of a 1d domain always had boundary indicator zero, the right boundary always boundary indicator one. This was different from the 2d/3d case where by default (unless you explicitly set things differently) all boundaries have indicator zero. This left-boundary-has-id-0, right-boundary-has-id-1 is still the default today, but at least you can set the boundary indicators of these end-points to something different today.

A second difference is that vertices have no extent, and so you can't apply quadrature to them. As a consequence, the FEFaceValues class wasn't usable in 1d. Again, this should work these days: every quadrature formula that has a single quadrature point is a valid one for points as well.

### I want to use deal.II for work in my company. Do I need a special license?

Before going into any more details, you need to carefully read the license deal.II is under. In particular, the explanations below are not meant to be legal advice and does not override the provisions in the Open Source license.

However, before this, let us provide our overarching philosophy: It is our intention to have constructive relationships with those who want to use our work commercially, and we encourage commercial use. After having used a more restrictive license until 2013, we have come to the conclusion that these licenses serve neither side particularly well: it made commercial use difficult, and the lack of commercial use deprived us of critical feedback, potential contributions from professional users, and our users of potential employment opportunities. Everyone is better off with the LGPL license we are using now, and we hope that deal.II also finds use in commercial settings.

Now for the smaller print: Generally, the LGPL is a fairly liberal license. In particular, if you develop a code based on deal.II, then there is no requirement that you also open source your own code: you can keep it closed source, under a proprietary license, and you don't need to give it to anyone (neither your customers nor to us).

The LGPL is only restrictive in that the changes you make to deal.II itself must also be licensed under the LGPL. There is not frequently a need to change the library itself, and in many of these cases you will probably be interested to get them into the upstream development sources anyway (e.g., in cases of bugs) rather than having to forward port them indefinitely. Of course, we are interested in this as well. However, there is no such requirement that you upstream these changes: the only people you have to make these modifications to deal.II available to are your customers.

As mentioned above, the preceding paragraphs are not a legal interpretation. For definite interpretations of the LGPL, you may want to consult lawyers familiar with the topic or search the web for more detailed interpretations.

## Supported System Architectures

### Can I use deal.II on a Windows platform?

deal.II has been developed with a Unix-like environment in mind and it shows in a number of places regarding the build system and compilers supported. That said, there are multiple methods to get deal.II running if you have a Windows machine.

#### Run deal.II in the Windows Subsystem for Linux

Windows 10 has gained a compatibility layer for running Linux binaries natively on Windows. You can find more information on the Wikipedia page. This means you do not have to use virtualization, or dual boot any more to install a full-featured Linux distribution! We summarize the installation on a separate wiki page on Windows.

#### Run deal.II natively on Windows

Since deal.II 8.4.0 we have experimental support for Microsoft Visual Studio (2013 and 2015). See the separate page on Windows for more details.

#### Run deal.II through a virtual box

The simplest way to try out deal.II is to run it in a premade virtual machine. You can download the virtual machine for VirtualBox from http://www.math.clemson.edu/~heister/dealvm/ and run it inside windows.

Note that your experience depends on how powerful your machine is. More than 4GB RAM are recommended. A native installation of Linux is preferable (see below).

#### Dual-boot your machine with Ubuntu

The simplest way to install Linux as a Windows user is to dual-boot. Dual-boot means that you simply install a second operating system on your computer and you choose which one to start when you boot the machine. Most versions of Linux support installing themselves as a second operating system. One example is using the Ubuntu installer for Windows. This installer will automatically dual-boot your system for you in a safe and fully reversible manner. Simply follow the instructions on http://www.ubuntu.com/download/desktop/install-ubuntu-with-windows

If at some point in the future you wish to remove Ubuntu from your system, from the Windows program manager (add-remove programs in older versions and programs and features in newer versions) you can simply uninstall Ubuntu as you would any other program.

Note: The actual install file is linked through the text "Windows installer" in the first gray box. You will be prompted to donate to Ubuntu, which is entirely optional. You will also be prompted to use a different version of Ubuntu if you use Windows 8.

### Can I use deal.II on an Apple Macintosh?

Yes, at least on the more modern OS X operating systems this works just fine. deal.II supports native compilers shipping with XCode as well as gcc from Mac Ports.

The only issue we are currently aware of is that if deal.II is configured to interface with PETSc, then PETSc needs to be configured with the `--with-x=0` flag to prevent linking in the X11 libraries (you probably won't need them anyway). Installing with PETSc has a myriad of other problems, though we believe that we have a way to stably interface it. You may want to read through the PETSc-related entries further down, however.

### Does deal.II support shared memory parallel computing?

Yes. deal.II supports multithreading with the help of the http://www.threadingbuildingblocks.org Threading Building Blocks (TBB) library. It is enabled by default and can be controlled via the `DEAL_II_WITH_THREADS` configuration toggle passed to `cmake` (see the deal.II readme file).

### Does deal.II support parallel computing with message passing?

Yes, and in fact it has been shown to scale very nicely to at least 16,384 processor cores in a paper by Bangerth, Burstedde, Heister and Kronbichler. You should take a look at the documentation modules discussing parallel computing, as well as the step-40 tutorial program.

### How does deal.II support multi-threading?

deal.II will use multi-threading using several approaches:

1. some BLAS routines might be multi-threaded (typically using OpenMP). This can be controlled from the command line using OMP_NUM_THREADS (also see the entry in the FAQ below)
2. Many places in the library are parallelized using the Threading Building Blocks (TBB) library.

MPI_InitFinalize() has an optional third argument that specifies the number of threads to use for the TBB. The default is 1. This gets send to the TBB via a call to MultithreadInfo::set_thread_limit(). If you pass numbers::invalid_unsigned_int into MPI_InitFinalize (or if you don't use that class, call set_thread_limit directly) then TBB will use the maximum number of threads that makes sense (and you can limit it using DEAL_II_NUM_THREADS from the command line).

Also note that while our Trilinos wrappers support multi-threading, the PETSc wrappers do not support this at this time, so you need to run with one thread per process.

### My deal.II installation links with the Threading Building Blocks (TBB) but doesn't appear to use multiple threads!

This may be a quirky interaction with the GOTO BLAS :-( If you use Trilinos or PETSc, both of these require a BLAS library from your system, and the deal.II cmake configuration will make sure that it is linked with. The problem stems from the fact that by default, the GOTO BLAS will simply grab all cores of the system for its own use, and -- before your `main()` function even starts, allow the main thread to use only a single core. (For the technically interested: it sets the processor scheduling affinity mask, using `set_sched_affinity` to a single bit.)

When the TBB initialization runs, still before `main()` starts, it will find that it can only run on a single core and will consequently not be able to work on multiple tasks in parallel.

The solution to this problem is to forbid the GOTO BLAS to grab all processors for itself, since we spend very little time in BLAS anyway. This can be done by setting either the `OMP_NUM_THREADS` or `GOTO_NUM_THREADS` environment variables to 1, see http://www.tacc.utexas.edu/tacc-software/gotoblas2/faq .

## Configuration and Compiling

### Where do I start?

Have a look at the ReadMe instructions for details on how to configure and install the library with `cmake`.

### I tried to install deal.II on system X and it does not work

That does occasionally (though relatively rarely) happen, in particular if you work on an operating system or with a compiler that the primary developers don't have access to. In a case like this, you should ask for help on the mailing list. However, remember: If your question only contains the text "I tried to install deal.II on system X and it does not work" then that's not quite enough to figure out what is happening. Even though the people developing this software belong to the most able programmers in the universe (and a decent number of parallel universes), all of us need data to find errors. So, whatever went wrong, paste the error message into your email. If the error is from the `cmake` invocation, show us the error message that was printed on screen. If the error happens after configuring and during compiling, add lines from screen output showing the error to the mail.

### How do I change the compiler?

deal.II can be compiled by a number of compilers without problems (see the section prerequisites in the readme file). If `cmake` does not pick the right one, selecting another is simple, and described in a section in the cmake documentation.

### I can configure and compile the library but installation fails. What is going on?

If you configure with the default `CMAKE_INSTALL_PREFIX`, the library is configured to installed to `/usr/local` and this fails without superuser rights with an error message like

``````CMake Error at cmake/scripts/cmake_install.cmake:42 (FILE):
file cannot create directory: /usr/local/common/scripts.  Maybe need
``````

Please see the readme on how to pick an install directory with write access (for example some path below your home directory).

### I get warnings during linking when compiling the library. What's wrong?

On some linux distributions with particular versions of the system compiler, one can get warnings like these during the linking stage of compiling the library:

```````.L3019' referenced in section `.rodata' of
of /home/bangerth/deal.II/lib/lac/sparse_matrix.float.g.o
``````

While annoying, these warnings do not actually seem to indicate anything particularly harmful. Apparently, the compiler generates the same code multiple times in exactly the same form, and the linker is only warning that it is throwing away all but one of the copies. There doesn't seem to be way to avoid these warnings, but they can be safely ignored.

### I can't seem to link/run with PETSc

Recent deal.II releases support PETSc 3.0 and later. This works, but there are a number of things that can go wrong and that result in compilation or linker errors, as explained below. If your program links properly with PETSc support, it will very likely also produce the correct results.

If you get errors like this when trying to run step-17 of the tutorials, even though linking seems to have succeeded just fine:

``````   [make run
============================ Running step-17
shared object file: No such file or directory
make: *** [run](step-17]) Error 127
``````

this means is that while linking, the compiler could find the libpetsc.so library, but the executable can't find it when running. The reason is that we can tell the linker where to look, but the executable apparently did not remember this (this is the standard Unix behavior). What you have to do is to set the LD_LIBRARY_PATH to include the path to the PETSc libraries. For example, under `bash` you would have to do this:

``````   export LD_LIBRARY_PATH=/path/to/petsc/libraries:\$LD_LIBRARY_PATH
``````

If you do so, the Unix loader can query the environment variable for where to find this particular library when trying to run the executable, and running the program should succeed.

Similarly, if you get errors of the kind during linking

``````/home/xxx/deal.II/lib/libdeal_II.g.so: undefined reference to
`KSPSetInitialGuessNonzero(_p_KSP*, PetscTruth)'
/home/xxx/deal.II/lib/libdeal_II.g.so: undefined reference to
`VecAXPY(_p_Vec*, double, _p_Vec*)'
...
``````

then the compiler can't seem to find the PETSc libraries. The solution is as above: specify the path to those libraries via `LD_LIBRARY_PATH`.

#### Is there a sure-fire way to compile deal.II with PETSc?

Short answer is "No". The slightly longer answer is, "PETSc has too many knobs, switches, dials, and a kitchen sink too many for its own damned good. There is not a sure-fire way to compile deal.II with PETSc!". It turns out that PETSc is a very versatile machine and, as such, there is no shortage of things that can go wrong in trying to configure PETSc to work seamlessly with deal.II on a first attempt. We have all struggled with this, although it has become a lot better in recent years.

You can find instructions on how to install PETSc linked to from the deal.II ReadMe file, or going directly to http://www.dealii.org/developer/external-libs/petsc.html .

#### I want to use HYPRE through PETSc

Hypre implements algebraic multigrid methods (AMG) as preconditioners, for example the BoomerAMG method. AMGs are among the most efficient preconditioners available and they have also been shown to be scalable to thousands of processors. deal.II allows the use of Hypre through the PETScWrappers::PreconditionBoomerAMG class; it is used in `step-40`. Hypre can be installed as a sub-package of PETSc and deal.II can access it through the PETSc interfaces.

To use the Hypre interfaces through PETSc, you need to configure PETSc as discussed in http://www.dealii.org/developer/external-libs/petsc.html , and add the following switch to the command line: `--download-hypre=1`.

#### Is there a sure-fire way to compile dealii with SLEPc?

Happily, the answer to this question is a definite yes; that is, if you have successfully compiled and linked PETSc already.

The real trick here is that during configuration SLEPc will pull out PETSc's configuration and just does whatever that tells it to do. Detailed steps are discussed in http://www.dealii.org/developer/external-libs/slepc.html .

Once deal.II is compiled, it is worth to start by looking at the step-36 tutorial program to see how to get started using the interface with SLEPc.

Note: To use the solvers and other algorithms SLEPc provides it is absolutely essential to have your PETSc installation working correctly since they share the same vector-matrix (and other) data structures.

### Trilinos detection fails with an error in the file `Sacado.hpp` or `Sacado_cmath.hpp`

This is a complicated one (and it should also be fixed in more recent Trilinos versions). In the Trilinos file `Sacado_cmath.hpp`, there is some code of the form

```  namespace std
{
inline float acosh(float x)
{
return std::log(x + std::sqrt(x*x - float(1.0)));
}
...
}```

In other words, Sacado is putting things into namespace `std`. The functions it is putting there are functions that have been defined by the C99 standard but that didn't make it into the C++98 standard before; some of them are widely used. The problem is that these functions were later added to the standard and so if your compiler is new enough (e.g. GCC 4.5 and later) then the compiler's C++ standard library already contains these functions. Adding them again in this file then yields errors of the kind

``````/home/.../trilinos-10.4.2/include/Sacado_cmath.hpp: In function 'float std::acosh(float)':
/home/.../trilinos-10.4.2/include/Sacado_cmath.hpp:41:16: error: redefinition of 'float std::acosh(float)'
/usr/include/c++/4.5/tr1_impl/cmath:321:3: error: 'float std::acosh(float)' previously defined here
``````

The only useful way to avoid this error is to edit the Trilinos header file. To do this, find and open the file `include/Sacado_cmath.hpp` in the directory in which Trilinos was installed. Then change the block enclosed in

```  namespace std
{
...
}```

```#ifndef _GLIBCXX_USE_C99_MATH
namespace std
{
...
}
#endif```

What this will do is make sure that the new members of namespace `std` are only added if the compiler has not already done so itself.

### My program links with some template parameters but not with others.

deal.II has many types for whose initialization you need to provide a template parameter, e.g. `SparseMatrix<double>`. The implementation of these classes can typically be found in files ending `.templates.h`, e.g. `sparse_matrix.templates.h`. The corresponding `.cc` files, e.g. `sparse_matrix.cc`, essentially only provide the explicit instantiations of these classes for the most commonly used template parameters. Sometimes this is done by including a corresponding `.inst` file, e.g. `sparse_matrix.inst`.

If you want to use a data type with a template parameter for which there is an explicit instantiation, you only need to include the respective `.h` header file, e.g. `sparse_matrix.h`. If, however, you want to use a template parameter for which there is no explicit instantiation in the corresponding `.cc` file, you have to include the respective `.templates.h` file in order for your program to link successfully.

The reason for all of this is essentially a matter of reducing compilation time. As long as you use data types with template parameters for which there is an explicit instantiation - and this should be the case most of the time - you do not need to compile the respective (lengthy) .templates.h file every time you compile your code. If, however, you need to use an instance of e.g. `SparseMatrix<bool>`, you have to include the respective `.templates.h` file and you have to compile it along with the remaining files of your program every time.

### When trying to run my program on Mac OS X, I get image errors.

You may encounter an error of the form

``````dyld: Library not loaded: libdeal_II.g.7.0.0.dylib. Reason: image not found
``````

on OS X. This goes hand in hand with the following message you should have gotten at the end of the output of `./configure`:

``````     Please add the line
export DYLD_LIBRARY_PATH=\\$DYLD_LIBRARY_PATH:\$DEAL2_DIR/lib
to your .bash_profile file so that OSX will be
able to find the deal.II shared libraries when
``````

What happens is this: when you say "make all", all the deal.II files are compiled and linked into a library (called libdeal_II.g.7.0.0) which on Macs have the file ending .dylib. Then you go to examples/step-1 and compile your program, which uses all the functions and classes that have previously been put into this library.

Now the following happens: On most operating systems, the actual executable program (i.e. the file step-1 in your directory that resulted from compiling) does not contain any information that would indicate where the various libraries that it uses can be found. For example, the step-1 program does not know where the libdeal_II.g.7.0.0.dylib file is. This is just how most operating systems function. But when you want to execute the program, somehow the program has to know where the library it needs is located. On most unix-like operating systems, this is done by setting an "environment variable" -- on linux this would the variable "LD_LIBRARY_PATH", on Mac OS X it is "DYLD_LIBRARY_PATH".

So to let the operating system know where the library is located, you could type export DYLD_LIBRARY_PATH=\$DYLD_LIBRARY_PATH:/Users/renjun/deal.ii/lib every time before you want to execute the program. That would be cumbersome. A simpler way would be if this export command is executed every time when you open a new shell window. This can be achieved by putting this command in a file that is executed every time you open a shell window. Depending on what shell you use, these files are alternatively called .cshrc .bashrc .bash_profile or similar. I'm not quite sure which file is relevant for you, but you can try them one after the other by putting the text in there, closing the window, opening it again, and then trying to execute ./step-1 (or saying "make run" in this directory) and seeing whether that works.

## C++ questions

### What integrated development environment (IDE) works well with deal.II?

The short answer is probably: whatever works best for you. deal.II uses the build tool CMake, which can generate a project description for virtually every IDE. In the past, many of the main developers have used emacs (or even vi), but there are much better tools around today, such as eclipse, KDevelop, Xcode, QtCreator, all of which have been used by people using deal.II.

We have gathered some notes on using the following IDEs for deal.II:

• Eclipse
• KDevelop
• emacs: While we don't recommend using emacs any more, this link provides a couple of notes on formatting styles used within deal.II.

When thinking about what IDE to use, keep this in mind: Many of us have used emacs (or, worse, vi) for years and feel very comfortable with it. But, emacs and vi were both started in 1976, at a time when computers had little memory, virtually no CPU power, and only text-based interfaces. While they have of course become a lot better over time, the design limitations this involved are still very much part of the code base: fundamentally, they are both still text-based and file-oriented. What IDEs can provide are multiple views of the same project in graphical and textual form and, more importantly, can integrate entire projects spanning hundreds of files in multiple directories: they know where a variable is declared (even if it's in a different file), what it's type is, and the properties of this type. Neither emacs nor vi nor any other older editor can provide anything that comes even close to what kdevelop or eclipse can offer in this regard.

What all this implies is that you should consider using one of the more modern tools, even if you're well acquainted with an existing, older one. Of course it takes a while to get used to a new application but my (Wolfgang's) experience with switching from emacs to kdevelop was that I have become '''so''' much more productive by using modern tools that the time invested in learning it was amortized very quickly. I found this experience a real eye-opener!

### Is there a good introduction to C++?

There are of course many good books and online resources that explain C++. As far as websites are concerned, www.cplusplus.com has both reference material for individual classes of the C++ standard library as well as a a tutorial on parts of the C++ language if you want to brush up on the correct syntax of things.

### Are there features of C++ that you avoid in deal.II?

There are few things that we avoid as a matter of principle. C++ is, by and large, a pretty well designed language in the sense that its features are there because they have been found to be useful by a lot of people. As an example, people have found that it is easier to write and debug code that throws exceptions in error cases rather than encoding error situations by special return values (e.g. by returning -1). There are of course ways to avoid exceptions (or templates, or certain parts of the C++ standard libraries, or any number of other things people have found objectionable in C++) and some software projects have chosen to restrict the use of C++ (for example Mozilla) or to emulate only those parts of C++ they like in C (e.g. the GNOME desktop environment, which leads to awkward to understand code as described here).

But ultimately, it is our belief that these approaches shoot their inventors in the foot: they avoid features of C++ that were really intended to make programming life simpler. It may be simpler for novice programmers to read code without templates; ultimately, however, learning to read and use templates will make you a much more productive programmer since you don't write the same code multiple times. As a consequence, the use of C++ is driven by the question of what is best suited to write a particular algorithm, not by abstract considerations. This fits into the realization that deal.II is a large piece of software -- not a small research project -- that requires professional software management practices and for which long term development can no longer be driven by an individual programmer's preferences of style.

### Why use templates for the space dimension?

The fundamental motivation for this is to use dimension-independent programming, i.e. you want to write code in such a way that it looks exactly the same in 2d as in 3d (or 1d, for that matter). There are of course many ways to do this (and libraries have done this for a long time before deal.II has). The three most popular ones are to use a preprocessor `#define` that sets the space dimension globally, to use a global variable that does this, and to have each object have a member variable that denotes the space dimension it is supposed to live in (in much the same way as the template argument does in deal.II). Neither of these approaches is optimal (nor is our own approach to use templates), however. In particular, using a preprocessor symbol or a global variable will not allow you to mix and match objects of different dimensionality. There are situations when you want to do that; for example deal.II internally builds higher dimensional quadrature formulas as tensor products of lower dimensional ones, and in application codes you may wish to discretize both volume models (e.g. simulating 3d models of plate tectonics and mountain belt formation) with surface models (e.g. erosion processes on the 2d earth surface).

This leaves the option to have a member variable denoting the space dimension in each object, a choice most other finite element libraries have followed. But this isn't optimal either, for two reasons. For example, consider this code that describes the equivalent of the `Point<dim>` class for points in dim-dimensional space and its `norm()` member function:

```class Point
{
public:
Point (const unsigned int dimension)
: dim(dimension),
coordinates (new double[dim])
{}

~Point() { delete[] coordinates; }

double norm () const;
...
private:
unsigned int dim;
double *coordinates;
};

double Point::norm () const
{
double s = 0;
for (unsigned int d=0; d<dim; ++d)
s += coordinates[d] * coordinates[d];
return std::sqrt(s);
}```

This is going to lead to rather slow code, for multiple reasons:

• The constructor and destructor have to allocate and deallocate memory on the heap, both expensive processes.

• When accessing any element of the `coordinates` array, two pointers have to be dereferenced. For example, the access to `coordinates[d]` really expands to `*(this->coordinates + d)`.

• The compiler can not optimize the loop since the upper bound `dim` of the loop variable is unknown at compile time.

Compare this to the way deal.II (approximately) implements this class:

```  template <int dim>
class Point
{
public:
Point () {}
~Point() {}

double norm () const;
...
private:
double coordinates[dim];
};

template <int dim>
double Point<dim>::norm () const
{
double s = 0;
for (unsigned int d=0; d<dim; ++d)
s += coordinates[d] * coordinates[d];
return std::sqrt(s);
}```

Here, the following holds:

• Constructor and destructor do not have to allocate and deallocate memory on the heap; rather, since the size of the `coordinates` array is known at compile time (i.e. whenever you instantiate the template for a particular dimension), the array lives on the stack. It is also much smaller than before: the dimension is encoded in the type and doesn't need a memory location, we don't need to store a pointer to an array, and we don't incur the memory overhead of having to manage an object on the heap.

• When accessing any element of the `coordinates` array, only one pointer has to be dereferenced. For example, the access to `coordinates` really expands to `*(this + d)`.

• The compiler can optimize the loop since the upper bound `dim` of the loop variable is known at compile time. In particular, for a point in 2d, the code the compiler will produce is likely to look more like this because the loop can be unrolled and the loop counter can be optimized away:

```double Point<2>::norm () const
{
return std::sqrt(coordinates[0] * coordinates[0] + coordinates[1] * coordinates[1]);
}```

Obviously, for a 3d point, the code will look differently, but the compiler can do this since it knows what the dimension of the point is at compile time.

There is another reason for the deal.II way: type safety. In short, a 2d point is not the same as a 3d point. If you assign one to the other, then this may be on purpose and the executable should simply change the value of the `dim` member variable from 2 to 3. But it may also be a legitimate error -- for example, you shouldn't be able to use 2d points to initialize the 3d quadrature points needed to integrate on a 3d cell. This can of course be caught by run-time checks, but the reason for strongly typed languages such as C++ has always been that it is much more efficient if the compiler can already catch this sort of error at compile time. Using templates for the space dimension avoids these sort of mistakes up front by forcing the programmer to explicitly specify her intent, rather than encoding intent in assertions.

Of course there are also downsides to using templates. Most notably, error messages that involve templates are notoriously unreadable, and that compiling template heavy code is slow: for example, we have to compile the `Point` class three times (for dim=1, dim=2 and dim=3) rather than only once. Nevertheless, we believe that these valid objections do not outweigh the benefits of templates.

### Doesn't it take forever to compile templates?

Yes, in general it does. The reason is that while for non-templates it is enough to put the ''declaration'' of a function into the header file and the ''definition'' into the `.cc` file, for templates that doesn't work. Let's say you have something like

`  template <typename T> T square (const T & t);`

`  template <typename T> T square (const T & t) { return t*t; }`

into the `.cc` file, then the compiler will say "Yes, I saw this template, and if I see a use of this function later on I will generate a function from it by replacing `T` by whatever type you use in the call". But if there is no call later on in the same `.cc` file, then the compiler won't do anything. If, at the same time, in a different `.cc` file that includes the header file, you use the function with `T=double` the compiler will say "Yes, I saw the declaration, but there is no definition; I assume the function has been compiled in a different `.cc` file with `T=double` and I'll simply record a call to this instantiation in the object file". The call will then be resolved at link time if indeed another object file contains an instantiation of the template for `T=double`. However, if no other object file contains such a definition, a linker error will result.

In general, for functions like the above, it is difficult to foresee what kinds of template arguments the function may be instantiated for, and so there is no real practical way to put the definition into a `.cc` file. Rather, one puts it into a header file, and so all `.cc` files that may use this function see its definition (i.e. its body) and the compiler can instantiate it in each source file for whatever template argument is necessary. This makes sure that you never get linker errors, but at the same time it makes compiling slow since every header file now not only has to parse the function's declaration, but also its definition -- and in the case of deal.II the definitions of all template functions add up to tens or hundreds of thousands of lines of code. This is one of the reason why many C++ programs compile relatively slowly: because they use a significant part of the C++ standard library, most of which consists of templates.

deal.II can avoid much of this overhead. The trick is to recognize that in the example above we don't really know what types `T` user code may possibly want to use for this template. But in the case of using the space dimension as a template parameter, we know pretty exactly all the possibly values: `Triangulation<dim>` may really only be instantiated for `dim=1, 2, 3` and for nothing else. Consequently, we can do the following: Put all the definitions of the member functions of deal.II into the `.cc` file and at the bottom of the file instruct the compiler to please instantiate all of these templates for `dim=1, 2, 3`. Similar things can be done for many other template functions in deal.II; for example, there are a good number of functions that require vector types as template arguments, of which deal.II provides a good number, yet this list is finite and enumerable. Consequently, we can simply, at the bottom of the `.cc` file, tell the compiler to instantiate all of these template functions for every single vector type deal.II supports, and then don't have to put thousands of lines of template definitions into header files.

In many cases, enumerating all possible template arguments is tedious; it is also difficult to extend this list when a new vector type is added, for example. To simplify this task, deal.II uses a preprocessor: for many files that want to instantiate a function or class for multiple template arguments, we have a file `.inst.in` that has the equivalent of a `for`-loop over all possible values or types for a template argument; the file is processed by the `common/scripts/expand_instantiations` program to produce a `.inst` file that can then be included into the `.cc` file.

### Why do I need to use `typename` in all these templates?

This is indeed a frequent question. To answer it, it is necessary to understand how a compiler deals with templates, which will take a bit of space here. Let's take for example this case:

```  void f(int);
void g(double d)
{
f(d);
}
void f(double);```

Here, in the function `void g(double)`, we call `f` with a double as an argument. Because at that point the compiler has only seen the declaration of the first overload of `f`, it will convert the double `d` to an integer and call this first overload. The fact that a second overload was declared later does not change this situation, since it wasn't visible at the time the compiler parsed `g`.

Templates are designed to work essentially the same, but there are slight complications. Take this example:

```  void f(int);
void f(char);
template <typename T> void g(T t)
{
f(1.1);
f(t);
}
void f(double);```

In the first line of `g`, the same thing happens as before: the argument is cast to `int` and the first of the two overloads of `f` is called. But when the compiler sees the template, it doesn't know yet what type `T` actually represents, so there is no way to settle on one of the two functions `f` the compiler has seen before when deciding about the second line. In fact, the C++ standard says that because the type of the argument `t` in the call depends on the template type, determining what function to actually call should only happen at the time and place when the template is instantiated (this is called argument dependent name lookup or ADL). In other words, if below the code above we had this:

```  void h()
{
g(1.1);
}```

then in the instantiation of `g` the first call would be to `f(int)` (because the argument 1.1 does not depend on the type given in the template argument, and consequently only functions are considered that were seen before the definition of `g(T)`) whereas the second call to `f` would be to `f(double)` -- even though `f(double)` wasn't even declared at the place the compiler saw the call in the template (though it is available at the place where we instantiate `g<double>`) -- because the function call argument `t` has type `T` and therefore depends on the template argument.

Argument dependent lookup allows you to use function templates like `g` with your own data types. For example, you could have your own library that does

```  #include <f.h>

struct X { /* something */ };

void f (const X & x) { /* do something with the X */ }

void my_function()
{
X x;
g(x);
}```

Presumably the writer of the `g` function did not know about your own type `X` yet, but her code still works because you provided a suitable overload of `f` in your own code.

So ADL is clever and allows you to use templates in ways the author of the template did not anticipate. But it has a dark side: for every statement in your code, the compiler has to figure out whether it depends on the template types or not, and it needs in fact to know quite a lot about it. Take this example:

```  int p;
template <typename T>
void g(T t)
{
T::something * p;
f(p);
}```

Here, is the call to `f` dependent because `p` depends on the type `T`? If `f` is called with an argument of type `X` that is declared like this

```  struct X
{
typedef int something;
};```

then `T::something * p;` would declare a local variable called `p` that is of type pointer-to-int. On the other hand, if we had

```  struct X
{
static double something;
};```

then `T::something * p;` multiplies the variable `X::something` by the global variable `p` and ignores the result of the multiplication. The following call to `f` would then be non-dependent because the type of the (global) variable `p` does not depend on the template argument.

The example shows that the compiler can't know whether a call is dependent or not in a template it is just seeing unless we tell it that `T::something` is supposed to be a type or a variable or function name. To avoid this situation, C++ says: if a compiler sees `T::something` then this is a variable or function name unless it is prefixed by the keyword `typename` in which case it is supposed to be a type. In other words, the call to `f` here is going to be non-dependent:

```  int p;
template <typename T>
void g(T t)
{
T::something * p;
f(p);
}```

and instantiating `g` with the first example for `X` is going to lead to errors because `T::something` didn't turn out to be a variable. On the other hand, if we had

```  int p;
template <typename T>
void g(T t)
{
typename T::something * p;
f(p);
}```

then the call is dependent and will be deferred until the compiler knows the type of `T`.

### Why do I need to use `this->` in all these templates?

This is a consequence of the same rule in the C++ standard as discussed in the previous question, Argument Dependent Lookup of names (ADL). Consider this piece of code:

```  template <typename T>
class Base
{
public:
void f();
};

template <typename T>
class Derived : public Base<T>
{
public:
void g();
};

template <typename T>
void Derived<T>::g()
{
f();
}```

By the rules, when the compiler parses the function `Derived::g` (note that parsing happens before and independently of instantiating the function for a particular argument type `T`), it sees that the call to `f()` does not depend on the template type and so it looks for a declaration of such a function somewhere. In the example above, it doesn't find one (we'll come to this in a second), which will yield an error. On the other hand, in this code,

```  void f(); // global function

template <typename T>
class Base
{
public:
void f();
};

template <typename T>
class Derived : public Base<T>
{
public:
void g();
};

template <typename T>
void Derived<T>::g()
{
f();
}```

it would find the global function and so when instantiating the function for, say, `T=int`, you'd get a function `Derived<int>::g` that would call the global function `::f`. This may or may not be what you had in mind.

The question of course is why the compiler didn't record a call to `Base<T>::f` in `Derived<int>::g`? After all, the compiler knows that `Derived` is derived from `Base`. This has a lot to do with the fact that at the time of parsing the template, the compiler doesn't know for which template arguments the template will later be instantiated, and with explicit or partial specializations. Consider for example this code:

```  template <typename T>
class Base
{
public:
void f();
};

class X
{
public:
void f();
};

template <> class Base<int> : public X {};

template <typename T>
class Derived : public Base<T>
{
public:
void g();
};

template <typename T>
void Derived<T>::g()
{
f();
}```

Here, if you look at `Derived<T>::g`, the call to `f()` will be resolved to `Base<T>::f` for all possible types `T`, unless `T=int` in which case the call will be to `X::f`. The point is that at the time the compiler sees (parses) the template, it simply doesn't know yet what `T` is, and so ADL says: if the call is not dependent, find a non-dependent function to record (e.g. a global function) rather than trying to find a call in scopes you can't yet know will be relevant (e.g. `Base` or `X`). Likewise, in this code,

```  template <typename T>
class Base
{
public:
void f();
};

template <>
class Base<int>
{
public:
struct f {};
};

template <typename T>
class Derived : public Base<T>
{
public:
void g();
};

template <typename T>
void Derived<T>::g()
{
f();
}```

the meaning of `f()` changes depending on the template type: if `T=int`, it creates an object of type `Base<int>>::f` and then throws the object away again immediately. For all other template arguments `T`, it calls `Base::f`.

Given this longish description of how compilers look up names under the ADL rule, let's get back to the original question: If you have this code,

```  template <typename T>
class Base
{
public:
void f();
};

template <typename T>
class Derived : public Base<T>
{
public:
void g();
};

template <typename T> void Derived<T>::g()
{
f();
}```

how do you achieve that the call in `Derived::g` goes to `Base::f`? The answer is: Tell the compiler to defer the decision of what the call is supposed to do till the time when it knows what `T` actually is. And we've already seen how to do that: we need to make the call dependent on `T`! The way to do that is this:

```  template <typename T>
void Derived<T>::g()
{
this->f();
}```

Here, `this` is a pointer to an object of type `Derived<T>`, which is of course dependent. So the resolution of what the statement is supposed to represent is deferred until instantiation time; at that time, however, the compiler knows what the base class is (for example it knows if there are explicit specializations) and so it knows which base classes to look into in an attempt to find a function with the name `f`.

### Does deal.II require C++11 support?

The answer to this question depends on the version of deal.II that you are interested in using

#### deal.II version 9.0.0

As of version 9.0.0, deal.II requires C++11 support equivalent to that provided by GCC 4.8.0., which is, essentially, every new feature in C++11.

#### deal.II version 8.5.0 and previous

The current release of deal.II, 8.5.0, is compatible with the C++98 and C++03 standards, but some features (e.g., the `LinearOperator` class) are only available if your compiler supports a subset of C++11 features. GCC 4.6 and newer implement enough of C++11 for these features to be turned on. More exactly, we currently require the following language features to be present:

1. `auto`-typed variables
2. The `nullptr` keyword
3. Move constructors
4. The `declval` and `decltype` keywords
5. Lambda functions

while we do not use the following features:

1. Marking virtual functions as `override`
2. Some features of the `type_traits` header, such as `std::is_trivially_copyable`
3. Inheriting constructors
4. Template aliases

The deal.II documentation has a page dedicated to the issue of what parts of C++11 we use and how this works. For a more complete list of features we do and do not use see the GCC 4.6 C++11 compatibility page.

### Can I convert Triangulation cell iterators to DoFHandler cell iterators?

Yes. You can also convert between iterators belonging to different DoFHandlers as long as the are based on the identical Triangulation:

```Triangulation<2>::active_cell_iterator it = triangulation.begin_active();

DoFHandler<2>::active_cell_iterator it2 (&triangulation, it->level(), it->index(), &dof_handler);```

## Questions about specific behavior of parts of deal.II

### How do I create the mesh for my problem?

Before answering the immediate question, one remark: When you use adaptive mesh refinement, you definitely want the initial mesh to be as coarse as possible. The reason is that you can make it as fine as you want using adaptive refinement as long as you have memory and CPU time available. However, this requires that you don't waste mesh cells in parts of the domain where they don't pay off. As a consequence, you don't want to start with a mesh that is too fine to start with, because that takes up a good part of your cell budget already, and because you can't coarsen away cells that are in the initial mesh.

That said, there are essentially three ways to generate a mesh, all of which are discussed in significantly more detail in the step-49 tutorial program:

• For many standard geometries (square, cube, circle, sphere, ...) there are functions in namespace `GridGenerator` that can generate coarse meshes.
• If `GridGenerator` does not offer a mesh for the geometry you have, but if the geometry is simple, then you can often create one "by hand". Take a look, for example, at how we create the mesh in step-14 using the `Triangulation::create_triangulation` function. All you need to do is take a piece of paper, draw the geometry and a number of coarse cells that form quadrilaterals, identify the locations of vertices and the connectivity from cells to vertices, and pass the corresponding lists to the Triangulation. Something similar can be done for simple 3d geometries.
• If your geometry is truly complicated enough so that you can't draw a mesh by hand any more (i.e. if it requires more than, for example, 20-30 coarse mesh cells), then you'll need a mesh generator. For quadrilaterals and hexahedra, there aren't all that many mesh generators. gmsh, lagrit and cubit come to mind. The primary problem is that most mesh generators' output meshes aren't particularly coarse by default, so you may want to pay particular attention to this point when running the mesh generator. (This is relevant since deal.II is particularly good about creating adaptively refined meshes, but if your coarse mesh is already very large then you will likely not have a lot of resources left to adaptively refine it some more.) Once you have a mesh from a mesh generator, you would read it using the `GridIn` class, as demonstrated, for example, in step-5.
• As it was already mentioned, if you do need to work with a geometry for which all you have is a triangular or tetrahedral mesh, then you can convert this mesh into one that consists of quadrilaterals and hexahedra using the tethex program, see https://github.com/martemyev/tethex .

### How do I describe complex boundaries?

You need to define classes derived from the `Boundary` base class and attach these to particular parts of the boundary of the triangulation. The `Triangulation` class will then query your boundary object whenever it needs a new point on the boundary after mesh refinement.

In deal.II releases after 8.1, the way geometry is described has been made much more flexible. In particular, it is no longer only possible to describe the boundary, but it is also possible to describe where points in the interior lie. The step-53 tutorial program explains how this is done for a realistic example.

### I am using discontinuous Lagrange elements (`FE_DGQ`) but they don't seem to have vertex degrees of freedom!?

Indeed. And here's the reason: a vertex is an entity that is shared between different cells, i.e. it doesn't belong to one cell or another. If you have a shape function that is associated with it, then its support will extend to all of the cells that are adjacent to the vertex since no cell is different than any other cell. This is what happens, for example, with the `FE_Q(1)` element. The same is true, by the way, for degrees of freedom (and associated shape functions) that correspond to edges and faces between cells.

But that doesn't answer the question of discontinuous elements. There, you have functions that are interpolation polynomials whose support point happens to be located at the same position as the vertex, but the actual support of the shape function is restricted to a single cell. In other words, '''logically''' these shape functions belong to a cell, not a vertex or edge or face, since the latter are all shared between adjacent cells. What this leads to is that, for example for the `FE_DGQ(1)` element, you have

• `fe.dofs_per_vertex` is zero
• `fe.dofs_per_line` is zero
• `fe.dofs_per_face` is zero
• `fe.dofs_per_cell` is 4 in 2d and 8 in 3d. In other words, all shape functions are associated with the cell interior.

If this answer isn't quite satisfactory (because, after all, the shape functions are defined by interpolation at the location of the vertices), one could turn the question around: If you ask me for the degree of freedom associated with vertex 13, then I should ask you in return which one you have in mind since if there, say, four cells that meet at this vertex, then there will be 4 degrees of freedom defined there. Likewise, if you ask me for the value of the degree of freedom associated with vertex 13, then I should ask you in return which one as the function is discontinuous there and will have multiple values at the location of the vertex.

### How do I access values of discontinuous elements at vertices?

The previous question answered why DG elements aren't defined at the vertices of the mesh. Consequently, functions like `cell->vertex_dof_index` aren't going to provide anything useful. Nevertheless, there are occasions where one would like to recover values of a discontinuous field at the location of the vertices, for example to average the values one gets from all adjacent cells in recovery estimators.

So how does one do that? The answer is: Getting the values at the vertices of a cell works just like getting the values at any other point of a cell. You have to set up a quadrature formula that has quadrature points at the vertices and then use an FEValues object with it. If you then use FEValues::get_function_values, you will get the values at all quadrature points (i.e. vertices) at once.

Setting up this quadrature formula can be done in two different ways: (i) You can create an object of type `Quadrature` from a vector of points that you can initialize with the reference coordinates of the 2dim vertices of a cell; or (ii) you can use the `QTrapez` class that has its quadrature points in the vertices. In the latter case, however, you need to verify that the order of quadrature points is indeed the same order as the vertices of a cell and, if that is not the case, translate between the two numbering systems.

### Does deal.II support anisotropic finite element shape functions?

There is currently no easy-to-use support for this. It's not going to work for continuous elements because we assume that `fe.dofs_per_face` is the same for all faces of a cell.

It may be possible to make this work for discontinuous elements, though. What you would have to do is define a bunch of different elements with anisotropic shape functions and select which element to use on which cells, using the `hp::DoFHandler` to deal with using different elements on different cells. The part that's missing is to implement elements with anisotropic shape functions. I imagine that this wouldn't be too complicated to do since the element is discontinuous, but someone would have to implement it.

That said, you can do anisotropic refinement, which of course also introduces a kind of anisotropic approximation of your finite element space.

### The graphical output files don't make sense to me -- they seem to have too many degrees of freedom!

Let's assume you have a 2x2 mesh and a Q1 element then you would assume that output files (e.g. in VTK format) just have 9 vertex locations and 9 values, one for each of the 9 nodes of the mesh. However, the file actually shows 16 vertices and 16 such values.

The reason is that frequently output quantities in deal.II are discontinuous: it may be that the finite element in use is discontinuous to begin with; or that the quantity we want to output is defined on a cell-by-cell basis (e.g. error indicators) and therefore discontinuous; or that it is a quantity computed from a DataPostprocessor object that could be discontinuous. In order to not make things more complicated than necessary, deal.II always assumes that quantities are discontinuous, even if some of them may in fact be continuous. The problem is that all graphical formats want to see one value for each output field per vertex. But discontinuous fields have more than one value at the location of a vertex of the mesh. The solution to the problem is then to simply output each vertex multiple times -- with different vertex numbers but at exactly the same location, once for each cell it is adjacent to. In other words, in 2d, each cell has four unique vertices. The 2x2 mesh in the example therefore has 16 vertices (4 vertices for each of the 4 cells) and we output 16 values. Several of these vertices will have the same location and if the field is indeed continuous, several of the values will also be the same.

### In my graphical output, the solution appears discontinuous at hanging nodes

Let me guess -- you are using higher order elements? If that's the case, then the solution only looks discontinuous but isn't really. What's happening is that the solution is, in fact, a higher order polynomial (e.g., a quadratic polynomial) along each edge of a cell but because all visualization file formats only support writing data as bilinear elements we need to write data in a way that shows only a linear interpolation of this higher order polynomial along each edge. This is no problem if the two neighboring elements share the entire edge because then the linear interpolations from both sides coincide. However, if we have a hanging node, then the value at the hanging node appears to float above or below the linear interpolation from the longer side, like here (in the left picture, see the gap at the bottom in the blue green area, and around the top left in the greenish area; pictures by Kevin Dugan):

From this description you can already guess what the solution is: the solution is internally in fact continuous: even though we only show a linear interpolation on the long edge, the true solution actually goes through the "floating" node. All this is, consequently, just an artifact of the way visualization programs show data.

If this bothers you or it simply looks bad in your graphics, you can lessen the problem by not plotting just a linear interpolation on each cell but outputting the solution as a linear interpolation on a larger number of "patches" per cell (e.g., plotting 5x5 patches per cell). This can be done by using the `DataOut::build_patches` function with an argument larger than one -- see its documentation.

This all said, if you are in fact using a Q1 element and you see such gaps in the solution, then something is genuinely wrong. One possibility is that you forget to call `ConstraintMatrix::distribute` after solving the linear system, or you do not set up these constraints correctly. In either case, it's a bug if this happens with Q1 elements.

### When I run the tutorial programs, I get slightly different results

This is sometimes unavoidable. deal.II uses a number of iterative algorithms (e.g. in solving linear systems, but the adaptive mesh refinement loop is also an iteration if you think about it) where certain criteria are specified by comparing floating point numbers. For example, the CG method terminates the iteration whenever the residual drops below a certain threshold; similarly, we refine as many cells as are necessary to take care of a fraction of the total error. In both cases, the quantities that are compared are floating point numbers which are subject to floating point round off. The problem is that floating point round off depends on the processor (sometimes), compiler flags or randomness (if parallelization is involved) and consequently an a solver may terminate one iteration earlier or later, depending on your environment, than the one from which we produced our results. With a different solution typically come different refinement indicators and different meshes downstream.

In other words, this is something that simply happens. What should worry you, however, is if you run the same program twice and you get slightly different output. This hints at non-deterministic effects that one should investigate.

### How do I access the whole vector in a parallel MPI computation?

Note that this causes a bottleneck for large scale computations and you should try to use a parallel vector with ghost entries instead. If you really need to do this, create a TrilinosWrappers::Vector (or a PETScWrappers::Vector) and assign your parallel vector to it (or use a copy constructor). You can find this being done in step-17 if you search for "localized_solution".

### How to get the (mapped) position of support points of my element?

Option 1: The support points on the unit cell can be accessed using FiniteElement::get_unit_support_point(s) and mapped to real coordinates using Mapping::transform_unit_to_real_cell()

Option 2: DoFTools::map_dofs_to_support_points() maps all the support points at once.

Option 3: You can create a FEValues object using the support points as a Quadrature:

```Quadrature q(fe.get_unit_support_points());
FEValues fe_values (..., q, update_q_points);
...
```

## Debugging deal.II applications

### I don't have a whole lot of experience programming large-scale software. Any recommendations?

Yes. First, the questions of this FAQ already give you a number of good pointers for example on debugging. Also, a good resource for some of the questions mathematicians, scientists and engineers (who may have taken a programming course, but know little of the bigger world of software engineering) typically have, is the Software Carpentry page. That site is specifically targeted at people who may want to use scientific computing to solve particular applications, but have little or no formal training in dealing with large software. In other words, it is specifically written for people for an audience like the users of deal.II.

### Are there strategies to avoid bugs in the first place?

Why yes, good you ask. There are indeed techniques that help you avoid writing code that has bugs. By and large, these techniques go by the name defensive programming, and the idea is to get yourself into a mindset while programming that anticipates that you will make mistakes, rather than expecting that your code is correct and then reacting to the situation when it turns out that this isn't true. The point is that even the most experienced programmers do introduce a lot of bugs into their code; what makes them good is that they have strategies to find them quickly and systematically.

Below we show one of the most important lessons learned. A more complete list can be found in our code conventions page which has a collection of best practices including code snippets to show how they are used.

The single most successful strategy to avoid bugs is to make assumptions explicit. For example, assume for a second that you have a class that denotes a point in 3d space:

```  class Point3d
{
public:
double coordinate (const unsigned int i) const;
// ...more here...
private:
double coordinates[3];
};

double
Point3d::coordinate (const unsigned int i) const
{
return coordinates[i];
}```

Here, when we wrote the `coordinate()` function, we worked under the assumption that the index `i` is between zero and two. As long as that assumption is satisfied, everything is fine. The problems start when someone calls this function with an index greater than two -- the function will in that case simply return garbage, but that may not be immediately obvious and may only much later lead to weird results in your program. Inexperienced programmers will say "Why would I do that, it doesn't make any sense!". Defensive programming starts from the premise that this is something that simply will happen at one point in time, whether you want to or not. It's actually not very difficult to do, since all of us have probably written code like this:

```  Point3d point;
// ... do something with it
double norm = 0;
for (unsigned int i=0; i<=3; ++i)
norm += point.coordinate(i) ** point.coordinate(i);
norm = std::sqrt(norm);```

Note that we have accidentally used `<=` instead of `<` in the loop.

If we accept that bugs will happen, we should make it as simple as possible to find them. In the spirit of making assumptions explicit, let's write above function like this:

```  double
Point3d::coordinate (const unsigned int i) const
{
if (i >= 3)
{
std::cout << "Error: function called with invalid argument!" << std::endl;
std::abort ();
}
return coordinates[i]
}```

This has the advantage that an error message is produced whenever the function is called with invalid arguments, and for good measure we also abort the program to make sure the error message can really not be missed in the rest of the output of the program. The disadvantage is that this check will always be performed whenever the program runs, even if it is well tested and we are fairly certain that in all places where the function is called, indices are valid. To avoid this drawback, the C programming language has the `assert` macro, which expands to the code above by default, but that can be disabled using a compiler flag. deal.II provides an improved version of this macro that is used as follows:

```  double
Point3d::coordinate (const unsigned int i) const
{
Assert (i<3, ExcMessage ("Function called with invalid argument!"));
return coordinates[i];
}```

The macro expands to nothing in optimized mode (see below), and if it is triggered in debug mode it doesn't only abort the program, but also prints an error message and shows how we got to this point in the program.

Using assertions in your program is the single most efficient way to make assumptions explicit and help find bugs in your program as early as possible. If you are looking for some more background, check out the wikipedia articles on assertions, preconditions and postconditions, and generally the design by contract methodology.

### How can deal.II help me find bugs?

In addition to using the `Assert` macro introduced above, the deal.II libraries come in two flavors: debug mode and optimized mode. The difference is that the debug mode libraries contain a lot of assertions that verify the validity of parameters you may pass when calling library functions and classes; the optimized libraries don't contain these and are compiled with flags that instruct the compiler to optimize. This makes executables linked against the optimized libraries between 4 and 10 times faster. On the other hand, you will find that you will find 90% or more of your bugs by using the debug libraries because most bugs simply pass data to other functions that they don't expect or that don't make sense. The consequence is that you should always use debug mode when you are still developing your code. Only when it runs without bugs -- and under no circumstances any earlier -- should you switch to optimized mode to do production runs. One of the silliest things you can do is switch to optimized mode because you otherwise get an error you can't make sense of and that you don't know how to fix; certainly, if the library complains about something and you ignore it, nothing good can come out of the remainder of the run of your program.

You can switch between debug and optimized mode, at least for the example programs, by compiling the example with either `make debug` or `make release`. There are further instructions in the documentation describing how to set this up in your own codes.

### Should I use a debugger?

This question has an emphatic, unambiguous answer: Yes! You may get by for a while by just putting debug output into your program, compiling it, and running it, but ultimately finding bugs with a debugger is much faster, much more convenient, and more reliable because you don't have to recompile the program all the time and because you can inspect the values of variables and how they change. Learn how to use a debugger as soon as possible. It is time well invested.

Debuggers come in a variety of ways. On Linux and other Unix-like operating systems, they are almost all based in one way or other on the GNU Debugger (GDB). GDB itself is a tool that is driven by interactively typing commands; if you know your way around with it, it is quite usable but it is rather austere and unless you are already familiar with this style of debugging, don't learn it. Rather, you should either use a graphical front-end or, even better, a front-end to GDB that is integrated into an Integrated Development Environment (IDE). An example of the stand-alone graphical front-ends to GDB are DDD, a program that was the first of its kind on Linux for many years but whose development has pretty much ceased in the early 2000s; it is still quite a good program, though. Another example is KDbg, a GDB front-end for the KDE desktop environment.

As mentioned, a better choice is to use a debugger front-end that is integrated into the IDE. Every decent IDE has an integrated debugger, so you have your choice. A list of IDEs and how they work with deal.II is given in the C++ section of this FAQ.

### deal.II aborts my program with an error message

You are likely seeing something like the following:

``````--------------------------------------------------------
An error occurred in line <1223> of file </.../dealii/include/deal.II/lac/vector.h> in function
Number& dealii::Vector<Number>::operator()(dealii::Vector<Number>::size_type)
[with Number = double; dealii::Vector<Number>::size_type = unsigned int]
The violated condition was:
i<vec_size
The name and call sequence of the exception was:
ExcIndexRangeType<size_type>(i,0,vec_size)
Index 10 is not in the half-open range [0,10).

Stacktrace:
-----------
#0  ./deliberate-mistake: foo()
#1  ./deliberate-mistake: main
--------------------------------------------------------

``````

This error is generated by the following program:

```#include <deal.II/lac/vector.h>
using namespace dealii;

void foo ()
{
Vector<double> x(10);
for (unsigned int i=0; i<=x.size(); ++i)
x(i) = i;
}

int main ()
{
foo ();
}```

So what to do in a case like this? The first step is to carefully read what the error message actually says as it contains pretty much all the information you need. So let's take the error message apart:

• The first two lines tell you where the problem happened: in the current case, in line 1223 of file `/.../dealii/include/deal.II/lac/vector.h` in the function `Number& dealii::Vector<Number>::operator()(dealii::Vector<Number>::size_type)`. This is a function in the library, so you likely don't know what exactly it does and what to do with it, but there is more information to come.

• The second part is the condition that should have been true but wasn't, leading to the error: `i<vec_size`. The variables involved in this condition (`i,vec_size`) are local variables of the function, or member variables of the class, so again you may not be entirely familiar with them. But you can already gather some of the information: `i` likely is an index, which should have been less than the variable `vec_size` (which sounds a lot like the length of a vector); the assertion says that it should have been smaller, but that it wasn't actually.

• There is more information: The exception generated is of kind `ExcIndexRange<size_type>(i,0,vec_size)` and the additional information says `Index 10 is not in the half-open range [0,10)`. In other words, the variable `i` has value `10`, and `vec_size` is also ten. This should already give you a fairly good idea what is happening: the vector has size ten, and following C array convention, that means that only indices zero through nine are value, but ten is not.

• The final part of the error message -- the stack trace -- tells you how you got to this place: reading from the bottom, `main()` called `foo()` which called the function that generated the error.

Taken together, this information should allow you figure out in 80% of cases what was going on, and fix the problem. Here, it is that we used the condition `i<=x.size()` in the loop, rather than the correct condition `i<x.size()`. In the remaining 20% of cases, things might be more difficult. For example, `foo()` might be a large and difficult function, and you would need to know in which part of the function did we access an invalid index of the vector. Or `i` was an index computed from other variables and you'd need to find out why it got the invalid value. In these cases, you'll have to learn how to use a debugger such as gdb, and in particular how to move up and down in the call stack and to inspect local variables in your source code.

### The program aborts saying that an exception was thrown, but I can't find out where

deal.II creates two kinds of exceptions (in deal.II language): ones where we simply abort the program because you are doing something that can't be right (such as accessing element 11 of a 10-element vector; this results in what has been discussed in the previous question) and ones that use the C++ construct `throw` to raise an exception. The latter construct is used for things that can't be statically checked in debug mode because they may depend on values read from input files or on a status that may simply change from one run of the program to the next; consequently, they always need to be verified, not only in debug mode, and there is sometimes a way to work around it in a program. The typical case is trying to write to a file that can't be opened (e.g. because the directory/file you specified in a parameter file doesn't exist or because the file system has run out of disk space).

Most of the time, the exceptions deal.II throws are annotated with the location and function where this exception was raised, and if you use a `main()` function such as the one used starting in step-6, this information will be printed. However, there are also cases where this kind of information is not available and then it is often difficult to establish where exactly the problem is coming from: all you know is that an exception was thrown, but not where or why.

To debug such problems, two approaches have proven useful:

• Run your program in a debugger (see the question about debuggers above, as well as these videos showing how to use the debugger in 22c8e221823811aa1178b450171824af: http://www.math.tamu.edu/~bangerth/videos.676.8.html, http://www.math.tamu.edu/~bangerth/videos.676.25.html). You need to instruct the debugger to stop whenever an exception is thrown. If you work with gdb on the command line, then issue the command `catch throw` before starting the program and it will stop everytime the code executes a `throw` statement. Integrated development environments typically also have ways of switching this on. Note that not every exception that is thrown actually indicates an error -- sometimes, there are legitimate reasons to throw an exception and catch it in the calling function, so you may have to continue (resume) a number of times before finding the place where this happens.

• Debugging by subtraction: Starting at the end of your program, remove one function/code block after the other until your program runs through without aborting. For example, if your program looked like step-6, see if it runs through if you don't create graphical output in `run()`. If it does, then you know that the exception must have been thrown in the block of code you just removed. If the program continues to abort, then reduce the number of mesh refinement cycles to find out within which cycle the problem happens. If it happens in the very first cycle, then remove calling the linear solver. If the program now runs through, then the problem happened in the solver. If it still aborts, then it must have happened before the solver, for example in the assembly. Repeating this, you will be able to narrow down which statement caused the problem, and knowing where a problem happens is already more than half of what you need to know to fix it.

### I get an exception in `virtual dealii::Subscriptor::~Subscriptor()` that makes no sense to me!

The full text of the error message probably looks something like this (the stack trace at the bottom is of course different in your code):

``````An error occurred in line <103> of file </.../deal.II/source/base/subscriptor.cc> in function
virtual dealii::Subscriptor::~Subscriptor()
The violated condition was:
counter == 0
The name and call sequence of the exception was:
ExcInUse (counter, object_info->name(), infostring)
Object of class N6dealii15SparsityPatternE is still used by 5 other objects.
from Subscriber SparseMatrix

Stacktrace:
-----------
#0  /.../deal.II/lib/libdeal_II.g.so.7.0.0: dealii::Subscriptor::~Subscriptor()
#1  /.../deal.II/lib/libdeal_II.g.so.7.0.0: dealii::SparsityPattern::~SparsityPattern()
#2  /.../deal.II/lib/libdeal_II.g.so.7.0.0: dealii::BlockSparsityPatternBase<dealii::SparsityPattern>::reinit(unsigned int, unsigned int)
#3  /.../deal.II/lib/libdeal_II.g.so.7.0.0: dealii::BlockSparsityPattern::reinit(unsigned int, unsigned int)
#4  /.../deal.II/lib/libdeal_II.g.so.7.0.0: dealii::BlockSparsityPattern::copy_from(dealii::BlockCompressedSimpleSparsityPattern const&)
#5  ./step-6: NavierStokesProjectionIB<2>::setup_system()
#6  ./step-6: NavierStokesProjectionIB<2>::run(bool, unsigned int)
#7  ./step-6: main
``````

What is happening is this: deal.II derives a bunch of classes from the `Subscriptor` base class and then uses the `SmartPointer` class to point to such objects. `SmartPointer` is actually a fairly simple class: when given a pointer, it increases a counter in the `Subscriptor` base of the object pointed to by one, and when the pointer is reset to another object or goes out of scope, it decreases the counter again. (It can also records who points to this object.) If someone tries to delete the object pointed to, then the destructor `dealii::Subscriptor::~Subscriptor()` is run and checks that in fact the counter in this object is zero, i.e. that nobody is pointing to the object any more -- because if some pointer was still pointing to it, it would be a poor decision to delete the object as then the pointer would point to invalid memory. If the counter is nonzero, you get the error above: you are trying to delete an object that is still pointed to. In the case above, you try to delete a `SparsityPattern` object (that is, from the stack trace, a part of a block sparsity pattern) even though there is still a `SparseMatrix` pointing to it (we get this from the "Additional Information" field).

The solution in cases like these is to make sure that at the time you delete the object, no other objects still have pointers that point to it.

There is one rather frequent case that results in an error like the above and that is often difficult to understand: if an exception is thrown in some function and not caught, all local objects are destroyed in the opposite order of their declaration; if it isn't caught in the function that called the place where the exception was generated, its local variables are also destroyed, and so on. This automatic destruction of objects typically bypasses all the clean-up code you may have at the end of a function and can then lead to errors like the above. For example, take this code:

```void f()
{
SparseMatrix s;
SparsityPattern sp;
// initialize sp somehow
s.reinit (sp);
Vector v;
// build a linear system

solve_linear_system (s, v);

s.reinit ();
}```

If the code executes normally, at the bottom of the function, the local variables `s,sp,v` will be destroyed in reverse order. Since we have called `s.reinit()`, the object no longer stores a pointer to `sp` and so destruction of `sp` before `s` incurs no harm. But if the function `solve_linear_system` throws an exception, for example because the linear system is singular, the call to `s.reinit()` isn't executed any more, and you will get an error like the one shown at the top.

In cases like these, the challenge becomes finding where the exception was thrown. The easiest way is to run your program in a debugger and let the debugger tell you whenever an exception is generated. In `gdb`, you can do that by saying `catch throw` before running the program; essentially, the command puts a breakpoint on all places where exceptions are thrown. Remember, however, that not every place where an exception is thrown is a candidate for the problem above: it may also be an exception that is caught in the function above and that never propagates to a point where it produces trouble. Consequently, it may well happen that you have to continue several times after seeing an exception thrown until you finally find the place where the offending exception happens.

### I get an error that the solver doesn't converge. But which solver?

Solvers are often deeply nested -- take a look for example at step-20 or step-22, where there is an outer solver for a Schur complement matrix, but both in the implementation of the Schur complement as well as in the implementation of the preconditioner we solve other linear problems which themselves may have to be preconditioned, etc. So if you get an exception that the solver didn't converge, which one is it?

The way to find out is to not wait till the exception propagates all the way to `main()` and display the error code there. Rather, you probably don't have a Plan B anyway if a solver fails, so you may want to abort the program if that happens. To do this, wrap the call to the solver in a try-catch block like this:

```  try
{
cg.solve (system_matrix, solution, system_rhs, preconditioner);
}
catch (...)
{
std::cerr << "*** Failure in Schur complement solver! ***" << std::endl;
std::abort ();
}```

Of course, if this is in the Schur preconditioner, you may want to use a different error message. In any case, what this code does is catch the exceptions thrown by the solver here, or by the system matrix's `vmult` function (if not already caught there) or by the preconditioner (if not already caught there). If you had already caught exceptions in the `vmult` function and in the preconditioner, then you now know that any exception you get at this location must have been because the CG solver failed, not the preconditioner, etc. The upshot is that you need to wrap every call to a solver with such a try-catch block.

### How do I know whether my finite element solution is correct? (Or: What is the "Method of Manufactured Solutions"?)

This is not always trivial, but there is an "industry-standard" way of verifying that your code works as intended, called the '''method of manufactured solutions'''. Before we describe the method, let us point this out: '''A code that has not been verified (i.e. for which correctness has not been established) is worthless. You do not want to have results in your thesis or a publication that may later turn out to be incorrect because your code does not converge to the correct solution!'''

The idea to verify a code is that you need a problem for which you know the exact solution. Unless you solve the very simplest possible partial differential equations, it is typically not possible to choose a right hand side and boundary values and then find the corresponding solution to the PDE analytically, on a piece of paper. But you can turn this around: Let's say your equation is Lu=f, then choose some function u and compute f=Lu. Note that the solution u does not necessarily have to be something that looks like a useful or physically reasonable solution to the equation, all that is necessary is that it is a function you know. Because L is a differential operator, computing f only involves computing the derivatives of the known function; this may yield lengthy expressions if you have nonlinearities or spatially variable coefficients in the equation, but should not be too complicated and can also be done using computer algebra programs such as Maple or Mathematica.

If you now put this particular right hand side f into your program (along with boundary values that correspond to the values of the function u you have chosen) you will get a numerical solution uh that we would hope converges against the exact solution u at a particular rate, say O(h2) in the L2 norm. But since you know the exact solution (you have chosen it before), you can compute the error between numerical solution and exact solution, and verify not only that your code converges, but also that it shows the convergence rate you expect.

The method of manufactured solutions is shown in the step-7 tutorial program.

### My program doesn't produce the expected output!

There are of course many possible causes for this, and you need to find out which of these causes might be the reason. Possible places to start are:

• Are matrix and right hand side assembled correctly? For most reasonably simple problems, you can compute the local contributions to these matrices by hand, and then compare those with the ones you compute on every cell of your program (remember that you can print the contents of the local matrix and right hand side to screen). A good strategy is also to reduce your problem to a 1x1 or 2x2 mesh and then print out the entire system matrix for comparison.

• Do you compute the matrix you need, or its transpose? The mathematical literature often multiplies the equation from the right with a test function but that is awkward because the matrix you get this way is the transpose from the one you need. The deal.II documentation goes to lengths in multiplying test functions from the left to avoid this sort of error; do the same in your derivations.

• Your constraints or boundary values may be wrong. While the ConstraintMatrix and functions like VectorTools::interpolate_boundary_values are well enough tested that they are unlikely candidates for problems, you may have computed constraints wrongly if you collect them by hand (for example if you deal with periodic boundary conditions or similar) or you may have specified the wrong boundary indicator for a Dirichlet boundary condition. Again, the solution is to reduce the problem to the simplest one you can find (e.g. on the 1x1 or 2x2 mesh talked about above) and to ask the ConstraintMatrix to print its contents so that you can compare it by hand with your expectations.

• Your discretization might be wrong. Some equations require you to use particular (combination of) finite elements; for example, for the Stokes equations and many other saddle point problems, you need to satisfy an LBB or Babuska-Brezzi condition. For other equations, you need to add stabilization terms to the bilinear form; for example, advection or transport dominated problems require stabilization terms such as artificial diffusion, streamlinear diffusion, or SUPG.

• The solver might be wrong. This can reasonably easily happen if you have a complex solver such as, for example, the one used in step-22. In such cases it has proven useful to simply replace the entire solver by the sparse direct UMFPACK solver (see step-29). UMFPACK is not the fastest solver around, but it never fails: if the linear system has a solution, UMFPACK will find it. If the output of your program is essentially the same as before, then the solver wasn't your problem.

• Your assumptions may be wrong. Double check that you had the correct right hand side to compute the numerical solution you compare against your analytical one. Also remember that the numerical solution is usually only an approximation of the true one.

In general, if your program is not computing the output you expect, here are a few strategies that have often worked in finding the problem:

• Take a good look at the output you get. For example, a close look can already tell you if (i) the boundary conditions are correct, (ii) the solution is continuous at hanging nodes, (iii) the solution follows the characteristics of the right hand side. This may already help you narrow down which part of the program may be the culprit. A common mistake is also to have a solution that by some accident is too large by a certain factor; consequently, the error will not converge to zero but to some constant value. This, again, is easily visible from a graphical representation of the solution and/or the error. Plotting the error is discussed in the section below entitled "How to plot the error as a pointwise function".
• If you have a time dependent problem, is the first time step right? There is no point in running the program for 1000 time steps and trying to find our why it is wrong, if already the first time step is wrong.
• If you still can't find what's going on, make the program as small as possible. Copy it to another directory and start stripping off parts that you don't need. For example, if it is a time dependent program for which you have previously already found out that the first time step is wrong, then remove the time loop. If you have tried whether you have the same problem when the mesh is uniformly refined, then throw out all the code that deals with adaptive refinement, constraints and hanging nodes. In this process, every time you simplify the program, verify that the problem is still there. If the problem disappears, you know that it must have been in the last simplification step. If the problem remains, it must be in the code that is now one step smaller. Ultimately, the code should be small enough so that you can just go through it and find the error by inspection.
• Learn to use a debugger. You will find that using a debugger is so much more convenient than trying to put screen output statements into your code, recompiling, and hoping that they reveal the problem. Modern integrated development environment as the ones discussed elsewhere in this FAQ have the debugger built-in, allowing you to use it seamlessly in your editing environment.

### The solution converges initially, but the error doesn't go down below 10-8!

First: If the error converges to zero, then you are basically doing something right already. Congratulations!

As for why the error does not converge any further, there are two typical cases what could be the reason:

• While the discretization error should converge to zero, the error of your numerical solution is composed of both the discretization error and the error of your linear or nonlinear solver. If, for example, you solve the linear system to an accuracy of 10-5, then there will be a point where the discretization error will get smaller than that by using finer and finer meshes but the solver error will not become smaller any more. To continue observing the correct convergence order, you will also have to solve the linear system with more accuracy.

• If you compute the error through an external program, for example by writing out the solution to a file and reading it from another program that knows about the exact solution, then you need to make sure you write the solution with sufficient accuracy. The default setting of C++ writes floating point numbers with approximately 8 digits, so if you want to make sure that your solution is correct to 10-10, for example, you'll have to write out the solution with more than 10 digits.

### My code converges with one version of deal.II but not with another

That is a tough case because the problem could literally be anywhere in the functions you call from deal.II. Rather than trying to start debugging blindly to find out what exactly is going on it's probably more productive to delineate the steps one could use to narrow down where the problem is.

In an ideal world, you would have already found out which commit in the history of deal.II caused the problem. Let's say you have checked out the two offending versions of deal.II into separate source directories `dealii-good` and `dealii-bad`, and that you compiled them both separately and installed them into directories `install-good` and `install-bad`. If you can't find out which commit caused the problem, the good and bad versions could also be the last two releases.

Let's also say that you have a directory `application` in which you have your own code. Now create two directories, `app-good`, `app-bad` parallel to `application`. Then do

```  cd application
for i in * ; do
ln -s \$i ../app-good/\$i
done```

This way you have two directories in which you can configure, compile, and run the exact same version of your application (exact same because they both contain links to the exact same source files), just compiled against the good and bad versions of the library, respectively.

So you do

```  cd app-good
cmake . -DDEAL_II_DIR=.../install-good
make

make```

If you run in these two directories, e.g., in two separate xterm windows, you will get one working and one failing run. Now start modifying the source files in `application` to figure out where the first point in the program is where there are differences. For example, after assembly, you could do insert a statement of the form

`  std::cout << "Linear system: " << system_matrix.l1_norm() << ' ' << system_rhs.l2_norm() << std::endl;`

I would suspect (though that doesn't have to be true -- but just assume for the moment) that if you compile and run this modification in your two windows that you will get different results. At this point, you can remove everything that is executed after this point from your program -- likely a few hundred lines of code. Or, if you're too lazy, just put `abort()` after that statement because everything that comes after it clearly only shows symptoms but not the cause of the problem.

Now that you know that the problem exists at the end of assembly, make your way further forward in the program. For example, is the local matrix on the first cell on which you assemble the same between the two programs? If it is, the problem is on a later cell. If it isn't the same, try to think about what the cause may be. Is the mesh the same? You can test that by putting output into an earlier spot of the program; if that output is different between the two programs, you can again delete everything that happens after that point.

The whole exercise is designed to find the first place in the program where you can unambiguously say that something has changed. Non-convergence is just such a non-specific problem that it is not helpful in finding what exactly is going on. It also happens rather late in typical programs that there are too many possibilities for where the root cause may be.

### My time dependent solver does not produce the correct answer!

For time dependent problems, there are a number of other things you can try over the discussion already given in the previous answer. In particular:

• If you have a time iteration and the solution at the final time (where you may evaluate the error) is wrong, then it was likely already wrong at the first time step. Try to run your program only for a single time step and make sure the solution there is correct. For example, it could be that you set the boundary values wrongly; this would be quite apparent if you looked at the first time step because the effect would be largest close to the boundary, but it may no longer be visible if you ran your program for a couple hundred time steps.

• Are your initial values correct? Output the initial values using DataOut just like you output the solution and inspect it for correctness.

• If you have a multi-stage time stepping scheme, are all the initial values correct?

• Finally, you can test your scheme by setting the time step to zero. In that case, the solution at time step zero should of course be equal to the solution at time step zero. If it isn't, you already know better where to look.

### My Newton method for a nonlinear problem does not converge (or converges too slowly)!

Newton methods are tricky to get right. In particular, they sometimes converge (if slowly) even though the implementation has a bug because all that is required for convergence is that the search direction is a direction of descent; consequently, if for example you have the wrong matrix, you may compute something that is a direction of descent, but not the full Newton direction, and so converges but not at quadratic order.

Here are a few considerations for implementing Newton's method for nonlinear PDEs:

• Try it with a linear program by removing all the nonlinearities in your problem. Your Newton iteration must converge in a single step, i.e. the Newton residual must be zero at the beginning of the second iteration. If that's not the case, something is wrong in your implementation.

• Newton's iteration will converge with optimal order for the problem R(u)=0, where R may be thought of as a residual, if you consistently compute the Newton residual i, R(uk)) and the Newton (Jacobian) matrix R'(uk). If you have a bug in either of the two, your method may converge, but typically at a (much) lower rate and with consequently many more iterations.

Consequently, one way to debug Newton's methods is to verify that the Newton matrix and Newton residual are matching in their code. However, if you have a matching bug in both of the matrix and right hand side assembly, then your Newton method will converge with correct order but against the wrong solution.

• If you have nonzero boundary values for your problem, set the correct boundary values for the initial guess and use zero boundary values for all following updates. This way, the updated uk+1 = uk + δ uk already has the right boundary values for all following iterations, where δ uk is the Newton update.

• If your problem is strongly nonlinear, you may need to employ a line search where you compute uk+1 = uk + α δ uk and successively try α=1, α=1/2, α=1/4, etc., until the residual computed for uk+1 for this α is smaller than the residual for uk.

• A rule of thumb is that if your problem is strongly nonlinear, you may need 5 or 10 iterations with a step length α less than one, and all following steps use the full step length α=1.

• For most reasonably behaved problems, once your iteration reaches the point where it takes full steps, it usually converges in 5 or 10 more iterations to very high accuracy. If you need significantly more than 10 iterations, something is likely wrong.

### Printing deal.II data types in debuggers is barely readable!

Indeed. For example, plain gdb prints this for cell iterators:

``````\$2 = {<dealii::TriaIterator<dealii::DoFCellAccessor<dealii::DoFHandler<2, 3> > >> = {<dealii::TriaRawIterator<dealii::DoFCellAccessor<dealii::DoFHandler<2, 3> > >> = {<std::iterator<std::bidirectional_iterator_tag, dealii::DoFCellAccessor<dealii::DoFHandler<2, 3> >, long, dealii::DoFCellAccessor<dealii::DoFHandler<2, 3> >*, dealii::DoFCellAccessor<dealii::DoFHandler<2, 3> >&>> = {<No data fields>},
accessor = {<dealii::DoFAccessor<2, dealii::DoFHandler<2, 3> >> = {<dealii::CellAccessor<2, 3>> = {<dealii::TriaAccessor<2, 2, 3>> = {<dealii::TriaAccessorBase<2, 2, 3>> = {
static space_dimension = <optimized out>, static dimension = <optimized out>,
static structure_dimension = <optimized out>, present_level = -9856,
present_index = 32767, tria = 0x4a1556}, <No data fields>}, <No data fields>},
static dimension = 2, static space_dimension = 3, dof_handler = 0x7fffffffdac8},
static dim = <optimized out>,
static spacedim = <optimized out>}}, <No data fields>}, <No data fields>}
``````

Fortunately, this can be simplified to this:

```\$3 = {
triangulation = 0x4a1556,
dof_handler = 0x7fffffffdac8,
level = 2,
index = 52
}```

All you need is (i) gdb version 7.1 or later, or a graphical frontend for it (e.g. DDD or kdevelop), (ii) some code that goes into your \$HOME/.gdbinit file. Instructions for setting up this file, which implements pretty printers for `Point`, `Tensor`, `Vector`, and the various iterator classes for triangulations and DoFHandlers, are posted here.

gdb can also pretty print many of the `std::XXX` classes, but not all linux distributions have it configured this way. To enable this, follow the instructions from this website. The little python snippet can be placed as a separate python block into `.gdbinit`.

### My program is slow!

This is a problem that is true for a lot of us. The question is which part of your program is causing it. Before going into more detail, there are, however, some general observations:

• Running deal.II programs in debug mode will take, depending on the program, between 4 and 10 times as long as in optimized mode. If you are using the standard setup for your own `CMakeLists.txt` file (described in the documentation), then compiling your code with `make release` will both compile your code at a higher optimization level and link it against the optimized version of deal.II.

• A typical finite element program will spend around one third of its time in assembling linear systems, around one half in solving these linear systems, and the rest of the time on other things. If your program's percentages significant deviate from this rule of thumb, you know where to start.

• There is a rule that says that even the best programmers are unable to point out where in the program the most CPU time is spent without some form of profiling. This is definitely true also for the primary developers of deal.II, so it is likely true for you as well. A corollary to this rule is that if you start optimizing parts of your code without first profiling it, you are more than likely just going to make things more complicated without significant gains because you pick the simplest places to optimize, not the ones with the biggest impact.

So how can you find out which parts of the program are slow? There are two tools that we've really come to like, both from the valgrind project: callgrind and cachegrind. Valgrind essentially emulates what your CPU would do with your program and in the process collects all sorts of information. In particular, if you run your program as in

``````  valgrind --tool=callgrind ./myprogram
``````

(this will take around 10 times longer than when you just call `./myprogram` because of the emulation) then the result will be a file in this directory that contains information about where your program spent its time. There are a number of graphical frontends that can visualize this data; my favorite is `kcachegrind` (a misnomer -- it is, despite its name, actually a frontend from callgrind, not cachegrind). Pictures of how this output looks can be found in the introduction of step-22. It typically shows how much time was spent in each function and a call graph of which functions where called from where.

Using valgrind's cachegrind can give you a more detailed look at much of the same kind of information. In particular, it can show you source line for source line how many instructions were executed there, and how many memory accesses (as well as cache hits and misses) were generated there. See the valgrind manual for more information.

Lastly, since you are probably most interested in the performance of the optimized version of your code (which you will probably use for long expensive runs), you should run valgrind on the optimized executable.

### How do I debug MPI programs?

This is clearly an awkward topic for which there are few good options: debugging parallel programs using MPI has always been a pain and it is frustrating even to experienced programmers. That said, there are parallel debuggers that can deal with MPI, for example TotalView that can make this process at least somewhat simpler.

Whether you have or don't have TotalView, here are a few guidelines of strategies that have helped us in the past:

• Try to reduce the problem to the smallest one you can find: The smallest mesh, the smallest number of processors. Reducing the number of processors needed to demonstrate the bug must be your highest priority.

• One of the biggest problems you typically have is that the processes that communicate via MPI typically run on different machines. If you can manage to reduce the problem to a small enough number of processors, you can run them all locally on a single workstation, rather than a cluster of computers. Ideally, you would reduce the problem to 2 or 4 processors and then just start the program using `mpirun -np 4 ./myexecutable` on the headnode of the cluster, a workstation, or even a laptop.

• Try to figure out which MPI process (the MPI rank) has the problem, for example by printing the output of `Utilities::System::get_this_mpi_process(MPI_COMM_WORLD)` at various points in your program.

• If you know which MPI process has the problem and if this is reproducible, let each process print out its MPI rank and its process id (PID) using the system function `getpid` at the very beginning of the program. The PID is going to be different every time you run the program, but if you know the connection between MPI rank and PID and you know which rank will produce the problem, then you can predict which PID will have the problem. The point of this is that you can attach a debugger to this PID; for example, `gdb` has the command `attach <pid>` with which you can attach the debugger to a running program, rather than running the program from the start within the debugger. Attaching a program to the debugger will stop it (and, after a while, will typically also stop all the other MPI processes once they come to a place where they are waiting for a communication from the stopped process). You can then look at variables, continue running to breakpoints, or do whatever else you want to do with the process you attached the debugger to. In particular, if for example you attached the debugger to the process that you know will segfault or run onto a failing assertion, you can just type `continue` in the debugger to let the program continue till it aborts. You can then inspect the state of the program at the point of the problem inside the debugger you attached.

• The above process relies on the fact that you have time to attach a debugger between starting the program, reading the mapping from MPI process rank to PID, and attaching a debugger. If the program produces the error very quickly, it is often useful to insert a call to `sleep(60);` (and including the appropriate header file) just after outputting MPI rank and PID. This gives you 60 seconds to attach the debugger before the program will continue.

• If finding out which MPI process has the problem turns out to be too complicated, or if it isn't predictable which process will produce an error, then there is a fallback option: attach a debugger to every MPI process. This is awkward to do by hand, but there is a shortcut: under linux (or any other unix system) you can run the program as in

``````  mpirun -np 4 xterm -e gdb --args ./my_executable
``````

The equivalent for macOS is

``````  mpirun -np 4 xterm -e lldb -f ./my_executable
``````

In this example, we start 4 MPI processes; in each of these 4 processes, we open an `xterm` window in which we start an instance of `gdb`/`lldb` with the executable. You'd then `run` the executable in each of the 4 windows, and debug it as you usually would. This might be tedious but as mentioned above, debugging MPI programs often is tedious indeed. To find out which gdb window belongs to which MPI rank, you can type the command

``````  !env | grep RANK
``````

### I have an MPI program that hangs

Apart from programs that segfault or that run onto a failing assertion (both cases that are relatively easy to debug using the techniques above), programs that just hang are the most common problem in parallel programming. The typical cause for this is that there is a point in your program where all or some MPI processes expect to get a message from a process X (e.g. in a global communication, say MPI_Reduce, MPI_Barrier, or directly in point-to-point communications) but process X is not where it should be -- for example, because it is in an endless loop, or -- more likely -- because process X didn't think that it should participate in this communication. In either case, the other processes will wait forever for process X's message and deadlock the program. An example for this case would go like this:

```  void assemble_system ()
{
// optimization in case there is nothing to do; we won't
// have to initialize FEValues and other local objects in
// that case
if (tria.n_locally_owned_active_cells() == 0)
return;

...
for (cell = ...)
if (!cell->is_ghost() && !cell->is_artificial())
...do the assembly on the locally owned cells...

system_rhs.compress();
}```

Here, the call to `compress()` at the end involves communication between MPI processes. In particular, say, it implies that process Y will wait for some data from process X. Now what happens if process X realizes that it doesn't have any locally owned cells? In that case, process X will quit the function at the very top, and will never call `compress()`. In other words, process Y will wait forever, possibly making process Z wait further down the program etc. In the end, the program will be deadlocked.

The goal of debugging the program must be to find where individual processes are stopped in order to determine which incoming communication they are waiting for. If you attached a debugger to the program above, you'd find for example that all but one process is stopped in the call to `compress()`, and the one remaining process is stopped in some other MPI call, then you already have a good idea what may be going on.

### One statement/block/function in my MPI program takes a long time

Let's say you have a block of code that you suspect takes a long time and you want to time it like this:

```  Timer t;
t.start();
my_function();
t.stop();
if (my MPI rank == 0)
std::cout << "Calling my_function() took " << timer() << " seconds." << std::endl;```

The output is large, i.e. you think that the function you called is taking a long time to execute and that you should focus your efforts on optimizing it. But in an MPI program, this isn't quite always true. Imagine, for example, that the function looked like this:

```  void my_function ()
{
double val = compute_something_locally();
double global_sum = 0;
MPI_Reduce (&val, &global_sum, MPI_DOUBLE, 1, 0, MPI_COMM_WORLD);
if (my MPI rank == 0)
std::cout << "Global sum = " << global_sum << std::endl;
}```

In the call to `MPI_Reduce`, all processors have to send something to processor zero. Processor zero will have to wait till everyone sends stuff to this processor. But what if processor X is still busy doing something else (stuff above the call to `my_function`) for a while? The processor zero will wait for quite a while, not because the operations in `my_function` are particularly expensive (either on processor zero or processor X) but because processor X was still busy doing something else. In other words: you need to direct your efforts in making the "something else on processor X" faster, not making `my_function` faster.

To find out whether this is really the problem, here is a simple way to see what the "real" cost of `my_function` is:

```  Timer t;
MPI_Barrier (MPI_COMM_WORLD);
t.start();
my_function();
MPI_Barrier (MPI_COMM_WORLD);
t.stop();
if (my MPI rank == 0)
std::cout << "Calling my_function() took " << timer() << " seconds." << std::endl;```

This way, you really only measure the time spent between when all processors have finished doing what they were doing before, and when they are all finished doing what they needed to do for `my_function`.

Another way to find some answers is to use the capabilities of the `Timer` class which can provide more detailed information when deal.II is configured to support MPI.

## I have a special kind of equation!

### Where do I start?

The deal.II tutorial has a number of programs that deal with particular kinds of equations, such as vector-valued problems, mixed discretizations, nonlinear and time-dependent problems, etc. The best way to start is to take a look at the existing tutorial programs and see if there is one that is already close to what you want to do. Then take that, try to understand its structure, and find a way to modify it to solve your problem as well. Most applications written based on deal.II are not written entirely from scratch, but have started out as modified tutorial programs.

### Can I solve my particular problem?

The simple answer is: if it can be written as a PDE, then this is possible as evidenced by the many publications in widely disparate fields obtained with the help of deal.II. The more complicated answer is: deal.II is not a problem-solving environment, it is a toolbox that supports you in solving a PDE by the method of finite elements. You will have to implement assembling matrices and right hand side vectors yourself, as well as nonlinear outer iterations, etc. However, you will not need to care about programming a triangulation class that can handle locally refined grids in one, two, and three dimensions, linear algebra classes, linear solvers, different finite element classes, etc.

To give only a very brief overview of what is possible, here is a list of the nontrivial problems that were treated by the programs that the main authors alone wrote to date:

• Time-dependent acoustic and elastic wave equation, including nonlocal absorbing boundary conditions;
• Stokes flow discretized with the discontinuous Galerkin finite element method;
• General hyperbolic problems including Euler flow, using the discontinuous Galerkin finite element method;
• Distributed parameter estimation problems;
• Mixed finite element discretization of a mortar multiblock formulation of the Laplace equation;
• Large-deformation elasto-plasticity in the simulation of plate tectonics.

To illustrate the complexity of the programs mentioned above we note that most of them include adaptive mesh refinement tailored to the efficient computation of specific quantities of physical interest and error estimation measured in terms of these quantities. This includes the solution of a so-called dual problems, that means e.g. for the wave equation the solution of a wave equation solved backward in time.

Problems other users of deal.II have solved include:

• Elastoplasticity;
• Porous media flow;
• Crystal growth simulations;
• Fuel cell simulations and optimization;
• Fluid-structure interaction problems;
• Time dependent large deformation problems for metal forming;
• Contact problems;
• Viscoelastic deformation of continental plates;
• Glacial ice flows;
• Thermoelastoplastic metal forming;
• Eulerian coordinates problems in biomechanical modeling.

Some images from these applications can be found on this wiki's Gallery page. A good overview of the sort of problems that are being solved with the help of deal.II can also be obtained by looking at the large number of publications written with the help of deal.II.

Probably, many other problem types are solved by the many users which we do not know of directly. If someone would like to have his project added to this page, just contact us.

### Why use deal.II instead of writing my application from scratch?

You can usually get the initial version of a code for any given problem done relatively quickly when you write it yourself, since the learning curve is not as steep as if you had to learn a new library; it's also true that it's easy to make this code twice as fast as if you had to use a library. In other words, this sounds like you should write finite element codes for your problem yourself.

However, you also need to keep in mind that it is the things you want to do after the first 3 months that will take you forever if you want to write it yourself, and where you will never be able to catch up with existing, established libraries: higher order elements; complicated, unstructured 3d meshes; parallelization; producing output in a format that's easily visualizable in 3d; adding an advected field for a tracer quantity; etc. Viewed this way, it's worth remembering that the primary commodity that's in short supply is not CPU time but your own programming time, and that's where you will be '''orders magnitude faster''' when using what others have already done, even if maybe your program ends up twice as slow as if you had written it from scratch with a particular application in mind.

### Can I solve problems over complex numbers?

Yes, you can, and it has been done numerous times with deal.II. However, we have a standard recommendation: consider such problems as systems of partial differential equations, where the individual components of the solution are the real and imaginary part of your unknown. The reason for this is that for complex-valued problems, the product `<u,v>` of two vectors is not the same as `<v,u>`, and it is very easy to get this wrong in many places. If you want to avoid these common traps, then the easiest way around is to split up you equation into two equations of real and imaginary part first, and then treat the resulting system as a system of real variables. This also makes the type of linear system clearer that you get after discretization, and tells you something about which solver may be adequate for it.

The step-29 tutorial program shows how this is done for a complex-valued Helmholtz problem.

### How can I solve a problem with a system of PDEs instead of a single equation?

The easiest way to do this is setting up a system finite element after you chose your base element, e.g.,

```FE_Q<dim> base_element(2);
FESystem<dim> system_element(base_element, 3);```

will produce a biquadratic element for a system of 3 equations. With this finite element, all the functions that you always called for a scalar finite element should just work for this vector-valued element as well.

Refer to the step-8 and in particular to the step-20 tutorial programs for a lot more information on this topic. Several of the other tutorial programs beyond step-20 also use vector-valued elements and there is a whole module in the documentation on vector-valued problems that is worth reading.

### Is it possible to use different models/equations on different parts of the domain?

Yes. The step-46 tutorial program shows how to do this: It solves a problem in which we solve Stokes flow in one part of the domain, and elasticity in the rest of the domain, and couple them on the interface. Similar techniques can be used if you want to exclude part of the domain from consideration, for example when considering voids in a body in which the governing equations do not make sense because there is no medium.

### Where do I start to implement a new Finite Element Class?

If you really need an element that isn't already implemented in deal.II, then you'll have to understand the interplay between FEValues, the finite element, the mapping, and quadrature objects. A good place to start would be to read the deal.II paper (Bangerth, Hartmann, Kanschat, ACM Trans. Math. Softw., 2007).

The actual implementation would most conveniently start from the `FE_Poly` class. You first implement the necessary polynomial space in the base library, then you derive `FE_Your_FE_Name` from `FE_Poly` (using your new polynomial class as a template) and add the connectivity information.

You'll probably need more specific help at various points -- this is what the mailing list is there for!

## General finite element questions

### How do I compute the error

If your goal is to compute the error in the form `||`u-uh`||` in some kind of norm, then you should use the function VectorTools::integrate_difference which can compute the norm above in any number of norms (such as the L2, H1, etc., norms). Take a look at step-7.

On the other hand, if your goal is to estimate the error, then the one class that can do this is Kelly Error Estimator. This class is used in most of the tutorial programs that use adaptively refined meshes, starting with step-6.

### How to plot the error as a pointwise function

The functions mentioned in the previous question compute the error as a cellwise value. As a consequence, the values computed also include a factor that results from the size of the cell. If you're interested in the pointwise error as something that can be visualized, for example because you want to find a pattern in why the solution is not as you expect it to be, what you should do is this:

• Interpolate the exact solution
• Subtract the interpolated exact solution from the computed solution
• Put the resulting vector into a DataOut object. This will plot the nodal values of the errors u-uh on the current mesh.

As an example, the following code shows how to do this in principle:

```  template <int dim>
class ExactSolution : public Function<dim>
{
public:
ExactSolution () : Function<dim>(dim+1) {}

virtual double value (const Point<dim>   &p,
const unsigned int  component) const
{
return ...exact solution as a function of p...
}
};

template <int dim>
void MyProblem<dim>::plot_error () const
{
Vector<double> interpolated_exact_solution (dof_handler.n_dofs());
VectorTools::interpolate (dof_handler,
ExactSolution<dim>(),
interpolated_exact_solution);
interpolated_exact_solution -= solution;

DataOut<dim> data_out;

data_out.attach_dof_handler (dof_handler);
}```

### I'm trying to plot the right hand side vector but it doesn't seem to make sense!

In particular, what you probably see is that the plot shows values that are smaller by a factor of two along the boundary than in the inside, and by a factor of four in the corners (in 2d) or eight (in 3d). Similarly, on adaptively refined cells, the values appear to scale with the cell size. The reason is that trying to plot a right hand side vector doesn't make sense.

While you plot the vector as if it is function (by connecting dots with straight lines in 1d, or plotting surfaces in 2d), the thing you right hand side vector is in fact an element of the dual space. To wit:

• A vector in primal space is a vector of nodal values so that `sum_i U_i phi_i(x)` is a reasonable function of `x`. Solution vectors are examples of elements of primal space.
• A vector in dual space is a vector W formed from the integration of an object in primal space against the shape functions, e.g. `W_i = int f(x) phi_i(x)`. Examples of dual vectors are right hand side vectors.

For vectors in dual space, it doesn't make sense to plot them as functions of the form `sum_i W_i phi_i(x)`. The reason is that the values of the coefficients W_i are not of amplitude kind. Rather, the W_i are of kind amplitude (e.g. f(x)) times integration volume (the integral `*` dx over the support of shape functions phi_i). In other words, the sizes of cells comes into play for W_i, as does whether a shape function lies in the interior or at the boundary. In your case, the area of the integral when you integrate against shape functions at the boundary happens to be half the size of the integration area for shape functions in the interior.

### What does XXX mean?

The documentation of deal.II uses many finite element specific terms that may not always be entirely clear to someone not familiar with this language. In addition, we have certainly also invented our shares of deal.II specific terminology. If you encounter something you are not familiar with, take a look at the deal.II glossary page that explains many of them.

## I want to contribute to the development of deal.II!

deal.II is Open Source -- this not only implies that you as everyone else has access to the source codes, it also implies a certain development model: whoever would like to contribute to the further development is invited to do so: If you have changes or ideas, please send them to the deal.II mailing list!

This model follows a small number of simple rules. The first and basic one is that if you have something that might be of interest to others as well, you are invited to send it to the list for possible inclusion into the library and use by others as well. Such additions useful to others are, for example:

• new backends for output in a new graphical format;
• input filters for some kind of data;
• tool classes that do something that might be interesting to use in other programs as well.

A few projects (some easy, some difficult) can also be found in the list of open issues, where they are generally marked as "Enhancements".

If you consider providing some code for inclusion with the library, these are the simple rules of gaining reputation in the Open Source community:

• your reputation grows with the number and complexity of your contributions;
• your reputation with the maintainers of the library also grows with the degree of conformance of your proposed additions with the administrative rules stated below;
• originators of code are credited full authorship.

In order to allow that a library remains a consistent piece of software, there are a number of administrative rules:

• there are a number of maintainers that decide what goes into the library;
• maintainers are benevolent, i.e. in general they want your addition to become part of the library;
• however, they have to evaluate additions with respect to some criteria, among which are value for others;
• whether it fits into the general framework (meaning that if your contribution requires the installation of some obscure other library that people do not usually have, then that must be discussed; alternatively, a way must be provided to disable your contribution on machines that do not have this lib);
• completeness and amount of documentation;
• existence and completeness of error checking through assertions.

However, again: the basic rule is that if you think your addition is interesting to others, there most probably is a way to get it into the library!

## I found a typo or a bug and fixed it on my machine. How do I get it included in deal.II?

First: thank you for wanting to do this! This software project is kept alive by people like you contributing to it. We like to include any improvement, even if it is just a single typo that you fixed.

If you have only a small change, or if this is your first time submitting changes, the easiest way to get them to us is by just emailing the deal.II mailing lists and we will make sure they get incorporated. If you continue submitting patches (which we hope you will!) and become more experienced, we will start to ask you to use git as the version control system and base your patches off of the deal.II github repository.

The process for this is essentially the following (if you don't quite understand the terminology below, take a look at the manuals at the github web site, read this online tutorial, or ask on the mailing list):

• Create a github account
• Fork the deal.II github repository, using the button at the top right of https://github.com/dealii/dealii
• Clone the repository onto your local file system
• Create a branch for your changes
• Create a pull request for your changes by going to your github account's deal.II tab where, after the previous step, there should be a button that allows you to create a pull request.

This list may sound intimidating at first, but in reality it's a fairly straightforward process that takes no more than 2 minutes after the first couple of times. But, as said, we'll be happy to hold your hand the first few times around and help you with the process! There's also a video lecture that demonstrates how to submit a patch to github.

If you've submitted patches several times and know your way around git by now, please also consider to

• make sure you base your patch off the most recent revision of the repository
• you rewrite the history of your patch so that it contains a relatively small number of commits that are each internally consistent and could also be applied independently (see, for example, the discussion towards the bottom of this page).

## I'm fluent in deal.II, are there jobs for me?

Certainly. People with numerical skills are a sought commodity, both in academia and in businesses. In the US, the National Labs are also hiring lots of people in this field.

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