Update 3

index.rst

@@ -20,7 +20,7 @@ Contents:
    matlab_plots
    matlab_exercises
    project_code_organization
-
+	oop
 
 
 Indices and tables

matlab_datatypes.mmd

@@ -224,6 +224,10 @@ The elements are accessed by name:
 	37
 ```
 
+One can have [structure arrays](http://www.mathworks.com/help/matlab/ref/struct.html), but they are tricky because each element must be a structure with the same fields.
+
+Often, [cell arrays](#cell-arrays) are more useful.
+
 #### Where structures are useful
 
 Use a structure to pass a large number of arguments to a function.
@@ -264,7 +268,24 @@ Example:
 
 ## Cell Arrays ##
 
-(http://www.mathworks.com/help/matlab/cell-arrays.html)
+A [cell array](http://www.mathworks.com/help/matlab/cell-arrays.html) is an n-dimensional array of mixed data.
+
+Each cell can hold a different data type.
+
+```matlab
+>> x = {'abc', 17; [3,4,5], {1, 2}}
+x = 
+    'abc'           [      17]
+    [1x3 double]    {1x2 cell}
+>> disp(x{1,1})
+abc
+>> disp(x{2,1})
+     3     4     5
+```
+
+Uses:
+
+* most common: replacement for structure array when one is not sure that all elements have the same fields.
 
 ## Numeric Precision ##
 
@@ -321,15 +342,4 @@ uint8
 
 > Rule of thumb: Store large datsets in low precision. Make everything double the moment you load it.
 
-## User Defined Data Types #
-
-It is possible to define new data types. See the Matlab documentation on [object oriented programming](http://www.mathworks.com/help/matlab/object-oriented-programming.html).
-
-`classdef` really defines a `struct` with certain required fields.
-
-Benefits: 
-
-* automatically ensures that each object has the right fields
-* can define `methods` (functions) that operate on the object.
-
 -----------

oop.mmd

@@ -0,0 +1,41 @@
+# Object Oriented Programming (OOP) #
+
+## The Idea ##
+
+OOP takes structured programming to the next level. Structured programming encapsulates local data in a function. The user does not need to know anything about the function other than the interface (inputs and outputs).
+
+OOP recognizes that some groups of functions "hang together" because they operate on the same object. One idea is to group these functions together.
+
+The second idea is that certain persistent data "belong to" an object. They should only be manipulated by functions that also "belong to" the object.
+
+OOP therefore bundles data (called `properties`) and functions (called `methods`) together.
+
+### Example: Utility function  ###
+
+$u(c,l) = c ^ (1-\sigma) / (1-\sigma) + \phi \log(l)$
+
+Persistent data include: parameters ($\sigma, \phi$).
+
+Methods include: compute $u_{c}$, $u(c,l)$, inverse marginal utility, indifference curves
+
+## Benefits ##
+
+There is nothing that OOP can do that could not be done without OOP. The benefits lie in code organization.
+
+The programmer sees all methods that operate on the object in one place. That makes it easier to 
+
+* test the code
+* modify the code
+* ensure consistency
+
+Since all code is in one place, it is easy to swap out.
+
+Imagine you want to compute a model with different utility functions. With OOP, all you need to do is swap out the utility function object. Ideally, the other code remains unchanged.
+
+## References ##
+
+Matlab documentation on [object oriented programming](http://www.mathworks.com/help/matlab/object-oriented-programming.html).
+
+---------
+
+