Structures and field access in MATLAB - Step-by-Step Execution

Concept Flow - Structures and field access

Create structure with fields

↓

Access field using dot notation

↓

Read or modify field value

↓

Use updated structure or field value

↓

End

This flow shows how to create a structure, access its fields using dot notation, read or change values, and then use the updated structure.

Execution Sample

MATLAB

person.name = 'Alice';
person.age = 30;
disp(person.name);
person.age = 31;
disp(person.age);

This code creates a structure 'person' with fields 'name' and 'age', displays the name, updates the age, and then displays the new age.

Execution Table

Step	Action	Structure State	Output
1	Create field 'name' with value 'Alice'	person.name = 'Alice'
2	Create field 'age' with value 30	person.name = 'Alice', person.age = 30
3	Display person.name	person.name = 'Alice', person.age = 30	Alice
4	Update person.age to 31	person.name = 'Alice', person.age = 31
5	Display person.age	person.name = 'Alice', person.age = 31	31
6	End of code	person.name = 'Alice', person.age = 31

💡 All lines executed; structure fields accessed and updated successfully.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
person.name	undefined	'Alice'	'Alice'	'Alice'	'Alice'
person.age	undefined	undefined	30	31	31

Key Moments - 3 Insights

Why do we use dot notation like person.name to access fields?

Dot notation specifies which field inside the structure we want. See execution_table step 3 where person.name is accessed to display 'Alice'.

What happens if we try to access a field that doesn't exist?

MATLAB gives an error because the field is not defined. In this example, all fields exist as shown in variable_tracker.

How do we update a field value inside a structure?

Assign a new value using dot notation, like person.age = 31 in step 4 of execution_table.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the output when displaying person.name?

A "Alice"

B "30"

C "person"

D Error

💡 Hint

Check the Output column at step 3 in execution_table.

At which step does person.age get updated to 31?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the Action column in execution_table where the age field changes.

If we tried to display person.height (not defined), what would happen?

A Displays empty string

B Error: Field 'height' does not exist

C Displays 0

D Displays 'height'

💡 Hint

Refer to key_moments about accessing undefined fields.

Concept Snapshot

Structure creation: s.field = value
Access field: s.field
Update field: s.field = new_value
Fields hold data of any type
Accessing undefined field causes error

Full Transcript

This example shows how to create a MATLAB structure named 'person' with fields 'name' and 'age'. We assign 'Alice' to person.name and 30 to person.age. Using dot notation, we display person.name which outputs 'Alice'. Then we update person.age to 31 and display it, outputting 31. Dot notation is used to access or modify fields inside the structure. Trying to access a field that does not exist will cause an error. The variable tracker shows how the fields change after each step. This helps beginners understand how structures store related data and how to work with their fields.

Practice

1. What is the correct way to access the field age from a structure person in MATLAB?

easy

A. person['age']

B. person.age

C. person->age

D. person.age()

Solution

1. Step 1: Understand MATLAB structure field access

   In MATLAB, fields of a structure are accessed using dot notation without parentheses or brackets.

2. Step 2: Identify correct syntax for field access

   The correct syntax is structure.field, so person.age is correct.

3. Final Answer:

   person.age -> Option B

4. Quick Check:

   Use dot notation for fields [OK]

Hint: Use dot notation to access structure fields in MATLAB [OK]

Common Mistakes:

• Using parentheses like a function call
• Using brackets like a dictionary
• Using arrow notation from other languages

2. Which of the following is the correct way to create a structure car with fields make and year in MATLAB?

easy

A. car = struct(make = 'Toyota', year = 2020);

B. car = {make: 'Toyota', year: 2020};

C. car = struct['make' => 'Toyota', 'year' => 2020];

D. car = struct('make', 'Toyota', 'year', 2020);

Solution

1. Step 1: Recall MATLAB struct creation syntax

   MATLAB uses the struct function with pairs of field names and values as strings and values.

2. Step 2: Match correct syntax

   car = struct('make', 'Toyota', 'year', 2020); uses struct('make', 'Toyota', 'year', 2020), which is correct MATLAB syntax.

3. Final Answer:

   car = struct('make', 'Toyota', 'year', 2020); -> Option D

4. Quick Check:

   Use struct('field', value, ...) to create structures [OK]

Hint: Use struct('field', value) pairs to create structures [OK]

Common Mistakes:

• Using JSON or dictionary syntax
• Using equal signs inside struct()
• Using brackets instead of parentheses

3. What is the output of the following MATLAB code?

person.name = 'Alice';
person.age = 30;
disp(person.age);

medium

A. 'Alice'

B. name

C. 30

D. Error: Undefined variable

Solution

1. Step 1: Understand structure field assignment

   The code assigns 'Alice' to person.name and 30 to person.age.

2. Step 2: Identify what disp(person.age) shows

   The disp function prints the value of person.age, which is 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   disp shows the value of the accessed field [OK]

Hint: disp(struct.field) prints the field's value [OK]

Common Mistakes:

• Confusing field names with values
• Expecting the field name instead of value
• Syntax errors from missing semicolons

4. Consider this MATLAB code snippet:

data = struct('x', 10, 'y', 20);
disp(data.z);

What will happen when this code runs?

medium

A. Error: Reference to non-existent field 'z'.

B. Displays 0

C. Displays empty array []

D. Displays 20

Solution

1. Step 1: Check structure fields

   The structure data has fields x and y, but no field z.

2. Step 2: Accessing a non-existent field

   Trying to access data.z causes MATLAB to throw an error about the missing field.

3. Final Answer:

   Error: Reference to non-existent field 'z'. -> Option A

4. Quick Check:

   Accessing missing fields causes an error [OK]

Hint: Access only existing fields; missing fields cause errors [OK]

Common Mistakes:

• Assuming missing fields return 0 or empty
• Ignoring error messages
• Confusing field names with variable names

5. You have a structure array students with fields name and score. How do you create a new structure passed containing only students with score >= 50?

hard

A. passed = students([students.score] >= 50);

B. passed = struct('name', students.name, 'score', students.score >= 50);

C. passed = students.score >= 50;

D. passed = students{students.score >= 50};

Solution

1. Step 1: Understand structure arrays and logical indexing

   In MATLAB, [students.score] creates an array of scores. Logical indexing selects elements where condition is true.

2. Step 2: Apply logical indexing to filter students

   The expression students([students.score] >= 50) selects only students with scores 50 or above.

3. Final Answer:

   passed = students([students.score] >= 50); -> Option A

4. Quick Check:

   Use logical indexing on structure arrays [OK]

Hint: Use logical indexing with [struct.field] to filter arrays [OK]

Common Mistakes:

• Trying to assign logical array directly
• Using curly braces instead of parentheses
• Creating new struct without filtering