MATLAB Programming/Arrays/Struct Arrays

Introduction to Structures
MATLAB provides a means for structure data elements. Structures are created and accessed in a manner familiar for those accustomed to programming in C.

MATLAB has multiple ways of defining and accessing structure fields. See Declaring Structures for more details.

Note: Structure field names must begin with a letter, and are case-sensitive. The rest of the name may contain letters, numerals, and underscore characters. Use the namelengthmax function to determine the maximum length of a field name.

Declaring Structures
Structures can be declared using the struct command. >> a = struct('b', 0, 'c', 'test') a = b: 0 c: 'test' In MATLAB, variables do not require explicit declaration before their use. As a result structures can be declared with the '.' operator. >> a.c = 'test' a = c: 'test' Structures can be declared as needed and so can the fields.

Arrays of Structures
Structures can also be arrays. Below is an example >> a = struct('b', 0, 'c', 'test');           % Create structure >> a(2).b = 1;                                % Turn it into an array by creating another element >> a(2).c = 'testing' a = 1x2 struct array with fields: b            c    >> a(1)                                        % Initial structure ans = b: 0 c: 'test' >> a(2)                                       % The second element ans = b: 1 c: 'testing'

Accessing Fields
When the field name is known the field value can be accessed directly. >> a.c       ans = test ans = testing In some cases you may need to access the field dynamically which can be done as follows. >> str = 'c'; >> a(1).(str) ans = test >> a(1).c       ans = test

Accessing Array Elements
Any given element in a structure array can be accessed through an array index like this >> a(1).c       ans = test To access all elements in a structure array use the syntax {structure.field}. In order to get all values in a vector or array use square brackets ([]) as seen below. >> [a.('c')] ans = testtesting >> [a.('b')] ans = 0    1

Or you can put them all into a cell array (rather than concatenating them) like this:

>> {a.('c')} ans = {'test', 'testing'}

Assigning values to a field of each struct array element
Matlab provides tools to assign values to a field of each array element. Consider the following struct array:

The following command assigns the same value to the field_b field of each array element:

To assign different values to each array element:

Sub-arrays through logical addressing
With Matlab, it's possible to extract a subarray from an array by using logical indexing. Consider the following struct array:

foo = struct('field_a',{1,2,3,4},'field_b',{4,8,12,16})

To obtain a subarray from foo where all foo.field_a values are equal to 2, a boolean array can be used to perform logical indexing. So, a boolean test that returns a boolean array for this purpose would be:

[foo.field_a] == 2

So, by using this boolean array to perform logical indexing, Matlab defines a struct array whose elements consist of those from foo whose field_a value is equal to 2 by doing:

foo([foo.field_a] == 2)