MATLAB - Structures

MATLAB - Structures

In MATLAB, a structure is a data type that groups related data using named containers called fields. Each field can hold any type of data, such as a number, a string, a matrix, or even another structure. This makes structures ideal for organizing complex, heterogeneous data, such as a patient record or an experiment’s parameters.

A structure array is an array where each element is a structure of the same format (i.e., having the same field names). This is incredibly useful for managing collections of similar items, like a list of students or a catalog of products.

Creating Structures

You can create structures in two primary ways: using dot notation or the struct function.

1. Using Dot Notation

Dot notation is the most intuitive way to create a single structure or add fields to an existing one. You simply assign a value to a field using the syntax structName.fieldName = value;.

% Create a structure to store information about one student
student.name = 'John Doe';
student.id = 12345;
student.courses = {'Calculus', 'Physics', 'History'};
student.grades = [85, 92, 78];

% Display the created structure
disp(student);

When you execute this code, MATLAB displays the structure and its contents:

  struct with fields:

        name: 'John Doe'
          id: 12345
     courses: {'Calculus', 'Physics', 'History'}
      grades: [85 92 78]

2. Using the struct Function

The struct function is ideal for creating a structure array from existing cell arrays or matrices. It is particularly efficient for pre-allocating structure arrays.

Syntax

s = struct(field1, value1, field2, value2, ...)

Here, fieldN is a character vector (e.g., 'Name') and valueN is typically a cell array of values for that field across all elements of the structure array.

Example: Creating a Structure Array of Students

% Define data in cell arrays
names = {'Riya', 'Tiya', 'Siya'};
ids = {101, 102, 103};
majors = {'Engineering', 'Arts', 'Science'};

% Create a 1x3 structure array
students = struct('Name', names, 'ID', ids, 'Major', majors);

% Access data for the second student
riyas_major = students(1).Major;
disp(riyas_major)

% Display the entire structure array
disp(students);

Output:

Engineering

1x3 struct array with fields:

    Name
    ID
    Major

You can inspect an individual element, for example students(2), to see its contents:

students(2)

ans =

  struct with fields:

     Name: 'Tiya'
       ID: 102
    Major: 'Arts'

Working with Structures and Structure Arrays

MATLAB provides several functions to manage and inspect structures.

Function	Description
fieldnames(S)	Returns a cell array of strings containing the names of the fields in structure S.
isfield(S, fieldName)	Checks if fieldName (a string) is a field in structure S. Returns logical true (1) or false (0).
isstruct(A)	Determines if the variable A is a MATLAB structure. Returns true or false.
rmfield(S, fieldName)	Removes the specified field fieldName from structure S and returns the modified structure.

Example: Using Structure Functions

% Re-use the 'students' structure array from the previous example
names = {'Riya', 'Tiya', 'Siya'};
ids = {101, 102, 103};
majors = {'Engineering', 'Arts', 'Science'};
students = struct('Name', names, 'ID', ids, 'Major', majors);

% 1. Get all field names
f_names = fieldnames(students);
disp('Field names:');
disp(f_names);

% 2. Check if 'ID' is a field (should be true)
has_id = isfield(students, 'ID');
disp(['Does struct have ''ID'' field? ', num2str(has_id)]);

% 3. Check if 'GPA' is a field (should be false)
has_gpa = isfield(students, 'GPA');
disp(['Does struct have ''GPA'' field? ', num2str(has_gpa)]);

% 4. Check if 'students' is a structure
is_a_struct = isstruct(students);
disp(['Is the variable a struct? ', num2str(is_a_struct)]);

% 5. Remove the 'Major' field
students_modified = rmfield(students, 'Major');
disp('Structure after removing ''Major'' field:');
disp(students_modified);

Output:

Field names:
    'Name'
    'ID'
    'Major'

Does struct have 'ID' field? 1
Does struct have 'GPA' field? 0
Is the variable a struct? 1
Structure after removing 'Major' field:
  1x3 struct array with fields:

    Name
    ID

Best Practice: Structures vs. Tables

For handling tabular data (where each row is an observation and each column is a variable), MATLAB’s table data type is often a more powerful and convenient alternative to a structure array. Tables offer column-based operations, metadata support, and seamless integration with statistical and plotting functions. Consider using a table when your data fits a spreadsheet-like format.