Programming and Data Types

Building Structure Arrays

You can build structures in two ways:

• Using assignment statements
• Using the struct function

Building Structure Arrays Using Assignment Statements

You can build a simple 1-by-1 structure array by assigning data to individual fields. MATLAB automatically builds the structure as you go along. For example, create the 1-by-1 patient structure array shown at the beginning of this section.

• patient.name = 'John Doe';
  patient.billing = 127.00;
  patient.test = [79 75 73; 180 178 177.5; 220 210 205];
  

Now entering

• patient
  

at the command line results in

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

patient is an array containing a structure with three fields. To expand the structure array, add subscripts after the structure name.

• patient(2).name = 'Ann Lane';
  patient(2).billing = 28.50;
  patient(2).test = [68 70 68; 118 118 119; 172 170 169];
  

The patient structure array now has size [1 2]. Note that once a structure array contains more than a single element, MATLAB does not display individual field contents when you type the array name. Instead, it shows a summary of the kind of information the structure contains.

• patient
  patient = 
  1x2 struct array with fields:
      name
      billing
      test
  

You can also use the fieldnames function to obtain this information. fieldnames returns a cell array of strings containing field names.

As you expand the structure, MATLAB fills in unspecified fields with empty matrices so that:

• All structures in the array have the same number of fields.
• All fields have the same field names.

For example, entering patient(3).name = 'Alan Johnson' expands the patient array to size [1 3]. Now both patient(3).billing and patient(3).test contain empty matrices.

Note    Field sizes do not have to conform for every element in an array. In the patient example, the name fields can have different lengths, the test fields can be arrays of different sizes, and so on.

Building Structure Arrays Using the struct Function

You can preallocate an array of structures with the struct function. Its basic form is

• str_array = struct('field1',val1,'field2',val2, ...)
  

where the arguments are field names and their corresponding values. A field value can be a single value, represented by any MATLAB data construct, or a cell array of values. All field values in the argument list must be of the same scale (single value or cell array).

You can use different methods for preallocating structure arrays. These methods differ in the way in which the structure fields are initialized. As an example, consider the allocation of a 1-by-3 structure array, weather, with the structure fields temp and rainfall. Three different methods for allocating such an array are shown in this table.

Method	Syntax	Initialization
struct	weather(3) =   struct('temp',72,'rainfall',0.0);	weather(3) is initialized with the field values shown. The fields for the other structures in the array, weather(1) and weather(2), are initialized to the empty matrix.
struct with repmat	weather =   repmat(struct('temp',72,   'rainfall',0.0),1,3);	All structures in the weather array are initialized using one set of field values.
struct with cell array syntax	weather = struct('temp',{68,80,72},   'rainfall',{0.2,0.4,0.0});	The structures in the weather array are initialized with distinct field values specified with cell arrays.

Structures

Accessing Data in Structure Arrays