Structures and field access in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Accessing nested structure fields

What is the output of the following MATLAB code?

MATLAB

person.name = 'Alice';
person.age = 30;
person.address.city = 'New York';
person.address.zip = 10001;
disp(person.address.city);

A Error: Field 'city' does not exist

B person.address.city

C 10001

D New York

💡 Hint

Use dot notation to access nested fields.

The code accesses the nested field 'city' inside 'address' of the 'person' structure, which holds the string 'New York'.

❓ Predict Output

intermediate

Modifying a structure field

What is the value of 'car.color' after running this code?

MATLAB

car.make = 'Toyota';
car.color = 'Red';
car.color = 'Blue';

A Error: Cannot modify field

B 'Red'

C 'Blue'

D '' (empty string)

💡 Hint

The last assignment changes the field value.

The field 'color' is first set to 'Red' and then changed to 'Blue'. The final value is 'Blue'.

❓ Predict Output

advanced

Accessing a non-existent field

What happens when this MATLAB code runs?

MATLAB

data.temperature = 25;
disp(data.humidity);

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

B Displays 0

C Displays empty array []

D Displays NaN

💡 Hint

Accessing a field that was never created causes an error.

MATLAB throws an error when trying to access a field that does not exist in the structure.

🧠 Conceptual

advanced

Structure array field access

Given the structure array 'students' below, what is the output of
disp(students(2).name)?

MATLAB

students(1).name = 'John';
students(1).grade = 85;
students(2).name = 'Emma';
students(2).grade = 92;

A John

B Emma

C 85

D Error: Index exceeds array bounds

💡 Hint

Use parentheses to access elements in a structure array.

students(2) accesses the second element of the structure array, and '.name' accesses the 'name' field, which is 'Emma'.

❓ Predict Output

expert

Dynamic field names in structures

What is the output of this MATLAB code?

MATLAB

fieldName = 'score';
result.(fieldName) = 100;
disp(result.score);

A 100

B fieldName

C Error: Invalid field name

D []

💡 Hint

You can use parentheses and a variable to access or create fields dynamically.

The code creates a field named 'score' dynamically using the variable 'fieldName' and assigns it 100. Displaying result.score shows 100.

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