Programming and Data Types

Accessing Data in Structure Arrays

Using structure array indexing, you can access the value of any field or field element in a structure array. Likewise, you can assign a value to any field or field element. For the examples in this section, consider this structure array.

You can access subarrays by appending standard subscripts to a structure array name. For example, the line below results in a 1-by-2 structure array.

mypatients = patient(1:2)

1x2 struct array with fields:
    name
    billing
    test

The first structure in the mypatients array is the same as the first structure in the patient array.

mypatients(1)

ans = 

       name: 'John Doe'
    billing: 127
       test: [3x3 double]

To access a field of a particular structure, include a period (.) after the structure name followed by the field name.

str = patient(2).name

str =

Ann Lane

To access elements within fields, append the appropriate indexing mechanism to the field name. That is, if the field contains an array, use array subscripting; if the field contains a cell array, use cell array subscripting, and so on.

test2b = patient(3).test(2,2)

test2b =

   153

Use the same notations to assign values to structure fields, for example,

patient(3).test(2,2) = 7;

You can extract field values for multiple structures at a time. For example, the line below creates a 1-by-3 vector containing all of the billing fields.

bills = [patient.billing]

bills =

  127.0000   28.5000  504.7000

Similarly, you can create a cell array containing the test data for the first two structures.

tests = {patient(1:2).test}

tests =

    [3x3 double]    [3x3 double]

Accessing Field Values Using setfield and getfield

Direct indexing is usually the most efficient way to assign or retrieve field values. If, however, you only know the field name as a string - for example, if you have used the fieldnames function to obtain the field name within an M-file - you can use the setfield and getfield functions to do the same thing.

getfield obtains a value or values from a field or field element

f = getfield(array,{array_index},'field',{field_index})

where the field_index is optional, and array_index is optional for a 1-by-1 structure array. The function syntax corresponds to

f = array(array_index).field(field_index);

For example, to access the name field in the second structure of the patient array, use:

str = getfield(patient,{2},'name');

Similarly, setfield lets you assign values to fields using the syntax

f = setfield(array,{array_index},'field',{field_index},value)

Building Structure Arrays

Finding the size of Structure Arrays