8.5: Structures

What Is a Structure?

A structure is another way to store related pieces of information together. The important difference is that a structure uses field names instead of numeric positions.

For example, suppose a professor wants to store information about a student: first name, last name, student ID number, and GPA. In a cell array, you would have to remember which column stores which information. In a structure, you can use meaningful field names.

student.FirstName = 'Elon';
student.LastName  = 'Musk';
student.ID        = 54343;
student.GPA       = 4.0;

Now the information is stored in a way that is easy to read. The expression student.GPA means the GPA field of the student structure.

Key Idea

A structure groups related data together using named fields. Instead of asking, “What was stored in column 4?” you can ask for student.GPA or package.price.

 

Creating a Structure with Dot Notation

The simplest way to create a structure is to use dot notation. Dot notation means you write the structure name, then a dot, then the field name.

package.item_no = 123;
package.cost    = 19.99;
package.price   = 39.95;
package.code    = 'g';

package

The first line creates a structure named package and gives it a field named item_no. The next lines add more fields to the same structure.

Dot notation is very readable, especially for beginners. The drawback is that it can become repetitive if you need to create many structures.

Creating a Structure with the struct Function

Another method is to use the struct function. This lets you define the field names and values in one statement.

package = struct('item_no', 123, ...
                 'cost',    19.99, ...
                 'price',   39.95, ...
                 'code',    'g');

The field names are written in quotes, followed by the value assigned to each field. This approach is compact and is especially helpful when creating arrays of structures.

Note

A single structure is treated as a 1-by-1 struct. That means package is one structure containing several fields.

 

Accessing Fields of a Structure

To access one field of a structure, use the structure name, a dot, and the field name. For example, 

package.cost

This returns the value stored in the cost field.

You can also store the value in another variable:

wholesaleCost = package.cost;

To see the names of all fields in a structure, use the fieldnames function.

fieldsOfPackage = fieldnames(package)

The result is a cell array of character vectors containing the field names.

 

Displaying a Structure

If you want to display a structure, you can type its name or use the disp function.

disp(package)

This displays the fields and their values. For small structures, this is a quick way to check that your data was stored correctly.

Exercise \(\PageIndex{1}\)

% Create a structure named sensor with fields ID, Type, Location, and Reading. 
% Then display the structure with its fields and values