Structure arrays in MATLAB - Step-by-Step Execution

Concept Flow - Structure arrays

Define structure fields

↓

Create structure with fields

↓

Create array of structures

↓

Access or modify elements

↓

Use structure array in code

You first define fields, then create structures with those fields, combine them into arrays, and access or change elements as needed.

Execution Sample

MATLAB

person(1).name = 'Alice';
person(1).age = 30;
person(2).name = 'Bob';
person(2).age = 25;

This code creates a structure array 'person' with two elements, each having 'name' and 'age' fields.

Execution Table

Step	Action	Structure Array State	Field Values	Notes
1	Assign person(1).name = 'Alice'	person(1)	name: 'Alice'	First element created with field 'name'
2	Assign person(1).age = 30	person(1)	name: 'Alice', age: 30	Added 'age' field to first element
3	Assign person(2).name = 'Bob'	person(1), person(2)	person(1): name='Alice', age=30; person(2): name='Bob'	Second element created with 'name'
4	Assign person(2).age = 25	person(1), person(2)	person(1): name='Alice', age=30; person(2): name='Bob', age=25	Added 'age' to second element
5	End	person(1), person(2)	person(1): name='Alice', age=30; person(2): name='Bob', age=25	Structure array fully defined

💡 All elements of the structure array have been assigned their fields.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
person	undefined	person(1).name='Alice'	person(1).name='Alice', person(1).age=30	person(1), person(2).name='Bob'	person(1), person(2).name='Bob', person(2).age=25	person(1).name='Alice', person(1).age=30; person(2).name='Bob', person(2).age=25

Key Moments - 3 Insights

Why does MATLAB create the structure array element-by-element instead of all at once?

Because each assignment like person(1).name creates or updates that element, MATLAB builds the structure array step-by-step as shown in steps 1 to 4.

What happens if you assign a field to person(2) before person(1) exists?

MATLAB automatically creates person(1) with empty fields to fill the gap, but this can lead to unexpected empty fields. The execution table shows sequential creation to avoid this.

Can all elements have different fields in a structure array?

No, all elements share the same fields. If a field is missing in one element, MATLAB fills it with empty values. The execution table shows both elements have 'name' and 'age'.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of person(1).age after Step 2?

A 30

B undefined

C 25

D empty

💡 Hint

Check the 'Field Values' column at Step 2 in the execution table.

At which step does person(2) first appear in the structure array?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Structure Array State' column to see when person(2) is created.

If you assign person(3).age = 40 without assigning person(3).name, what happens to person(3).name?

A It copies person(2).name

B It causes an error

C It becomes empty

D It becomes 'unknown'

💡 Hint

Recall that all elements share the same fields; missing fields get empty values.

Concept Snapshot

Structure arrays in MATLAB hold multiple elements with the same fields.
Create elements by assigning fields one by one.
All elements share the same field names.
Access fields with dot notation and index.
Missing fields in elements are empty by default.

Full Transcript

This visual trace shows how MATLAB builds a structure array step-by-step. First, person(1).name is assigned 'Alice', creating the first element. Then person(1).age is added as 30. Next, person(2).name is set to 'Bob', creating the second element, followed by person(2).age as 25. The structure array now has two elements, each with 'name' and 'age' fields. Variables update after each step, and all elements share the same fields. Beginners often wonder why elements are created one at a time and what happens if fields are missing. The quiz questions check understanding of these points.