# 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:

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

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

```value = 1;
[foo.field_b] = deal(value)
```

To assign different values to each array element:

```value = {4,8,12,16};
[foo.field_b] = value{:}
```

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)
```