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A variable and dictionary in pure fortran for retaining any data-type and a fast hash-table dictionary.


This module consists of two separate modules which co-exist for maintenance and usage reasons.

First, the variable module which is a type-free variable that can contain any variable type, and any dimension as well.

Second, the dictionary module which contains a hash-table of variables that can contain any data-type allowed by the variable module.

Downloading and installation

Installing fdict requires a download of the library hosted at github at fdict@git.

Extract and create an setup.make file for compilation, a minimal setup.make file can look like this


Type make and a library called libfdict.a is created.
Subsequently the installation may be performed by:

make PREFIX=/papth/to/fdict install

which installs the required files (modules and libraries) to the folder.

To use the dictionary you need to add include statements for the modules as well as linking to the program.

To link fdict to your program the following can be used in a Makefile

FDICT_PATH  = /path/to/fdict/parent

The file may be included in projects which exposes the following definitions:


which may be used in functional codes to utilize the correct interfaces. This is mainly meant as a feature usable when the fdict interface and e.g. modules change names.

Controlling interface parameters

Typically not needed: allows for customization of different interfaces.

By default the number of dimensions allowed by the library is 3, i.e. there is no interface created for real a(:,:,:,:), etc. However, to accomodate arbitrary dimensions you can call a setup script which initializes a different number of dimensions, which can be controlled individually.

Run ./ to get options regarding the setup.

For instance, if you require interfaces for real and real(kind(0.d0)) up to 4 dimensions and all others up to 3 dimensions you can do this

# -A == all data-types, s = single, d = double
./ -A 3 -s 4 -d 4
# -R is a shorthand for both -s and -d
./ -A 3 -R 4


Using this module one gains access to a generic type variable which can contain any data format.
It currently supports the following data-types:

Type Precision format C-type which
type(variable_t) --- VAR
character(len=1) char a
integer selected_int_kind(4) short h
integer selected_int_kind(9) int i
integer selected_int_kind(18) long l
real selected_real_kind(p=6) float r
real selected_real_kind(p=15) double d
complex selected_real_kind(p=6) float complex c
complex selected_real_kind(p=15) double complex z
type(c_ptr) void * cp
type(c_funptr) (procedure) void * fp

Basically it is used like this:

use variable
integer :: a(3)
type(variable_t) :: v
a = 2
call assign(v,a)
a = 3
call assign(a,v)

Also the variable contains an abbreviation for assigning pointers to not copy data, but retain data locality:

integer, target :: a(3)
type(variable_t) :: v
a = 2
call associate(v,a)
a = 3
! Now v contains a = 3

To delete a variable do:

use variable
type(variable_t) :: v
call delete(v)

However, when the variable is using pointers, instead the user can do

use variable
type(variable_t) :: v
! preferred
call nullify(v)
! or
call delete(v,dealloc=.false.)

which merely destroys the variable object and thus retains the data where it is. As with any other pointer arithmetic it is up to the programmer to ensure there is no memory leaks.

In some cases one does not know which data-type is being stored in a variable. Here it may be beneficial to lookup the type of data:

use variable
integer, target :: a(3)
type(variable_t) :: v
a(:) = 2
call associate(v,a)
if ( which(v) == 'i1' ) then ! signal integer of 1D (i0 for scalar)
   call assign(a, v)
end if

! Another possibility is to *try* to get the value
logical :: success
integer, target :: i1(3)
real, target :: r1(3)

call assign(r1, v, success=success)
if ( .not. success ) then
    call assign(i1, v, success=success)
end if
... etc ...

However, it may be better to explicitly check the type using which. The return values from which are listed in the above table.


Using type(variable_t) it becomes easy to create dictionaries in fortran.

Using this module we implement a dictionary which can contain any data format using a key:val based formalism. The underlying data structure is a linked list sorted according to hash-values of the keys. Hence searching for specific elements in the dictionary is extremely fast. Searching a dictionary with 100 keys 300000 times takes less than 0.04 seconds on a Haswell laptop. Concatenating dictionaries is also very fast.

Creating a dictionary is almost as easy as the Python equivalent:

use dictionary
type(dictionary_t) :: dict
dict = ('KEY'.kv.1)

To extend a dictionary one uses the concatenating format:

dict = dict // ('Hello'.kv.'world') // ('No'.kv.'world')

Again as is used by the type(variable_t) one can with benefit use .kvp. to create the dictionary value by pointers instead of copying the content.
Hence doing:

real :: r(4)
dict = dict // ('reals'.kvp.r)
r = 4

will change the value in the dictionary.
Note that one can easily create memory leaks with dictionaries:

use dictionary
type(dictionary_t) :: dict
dict = ('KEY'.kv.1)
dict = dict // ('KEY'.kv.2)
dict = ('KEY'.kv.3)

The 1st assignement is valid since the dictionary is empty. The 2nd assignment concatenates and does not produce any memory leaks. In that case the old key KEY is deleted and the new value 2 is inserted. The 3rd assignment produces a memory leak since the pointer to the original dictionary gets lost. Be sure to call call delete(dict) prior to single assignments.

There are various ways to access the data in a dictionary.

  1. Accessing specific keys may be exercised using

     use dictionary
     type(dictionary_t) :: dict
     type(variable_t) :: var
     integer :: i
     real :: r
     logical :: success
     dict = ('KEY'.kv.1)
     call assign(r, dict, 'KEY', success=success)
     if ( .not. success ) call assign(i, dict, 'KEY', success=success)
     call assign(var, dict, 'KEY')

    Since values in dictionaries are stored using variable_t we have to follow the limitations of that implementation. Therefore it may be better to always use a temporary variable_t to retrieve the values stored. This will remove a redundant lookup in the dictionary.

  2. Users may find the .key. and .value. operators which only acts on the first element of the dictionary (which may be a surprise). This is only useful for looping dictionaries.

     use dictionary
     type(dictionary_t) :: dict, dict_first
     type(variable_t) :: var
     character(DICTIONARY_KEY_LENGTH) :: key
     integer :: i
     real :: r
     logical :: success
     dict = ('KEY'.kv.1)
     dict = dict // ('KEY1'.kv.3)
     ! start looping
     dict_first = .first. dict
     do while ( .not. (.empty. dict_first) )
        ! now .key. and .value. could be used:
        key = .key. dict_first
        call assign(var, dict_first)
        ! Get next dictionary entry
        dict_first = .next. dict_first
     end while

Note that the dictionary can also contain any data type.

However, if it needs to do custom data-types the programmer needs to extend the code by supplying a few custom routines.

Intrinsically the dictionary can contain dictionaries by this:

use dictionary
type(dictionary_t) :: d1, d2
d1 = ('hello'.kv.'world')
d2 = ('hello'.kv.'world')
d1 = d1 // ('dict'.kvp.d2)

But it will be up to the user to know the key for data types other than integers, reals, complex numbers, characters and c_* extension types.

Note that the dictionary contained is passed by reference, and thus if you delete d2, you will have a dangling pointer in d1.

Contributions, issues and bugs

I would advice any users to contribute as much feedback and/or PRs to further maintain and expand this library.

Please do not hesitate to contribute!

If you find any bugs please form a bug report/issue.

If you have a fix please consider adding a pull request.


The fdict license is MPL-2.0, see the LICENSE file.


A big thanks goes to Alberto Garcia for contributing ideas and giving me bug reports. Without him the interface would have been much more complex!