Structures and field access in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a structure in MATLAB?

A structure in MATLAB is a data type that groups related data using named fields, allowing you to organize different types of data together like a real-world object.

beginner

How do you create a structure with fields 'Name' and 'Age' in MATLAB?

You can create it like this:
person.Name = 'Alice';
person.Age = 30;

beginner

How do you access the field 'Age' from a structure variable 'person'?

Use dot notation:
person.Age returns the value stored in the 'Age' field.

intermediate

How can you check if a field exists in a structure?

Use the function isfield(structure, 'fieldname'). It returns true if the field exists, false otherwise.

intermediate

What happens if you try to access a field that does not exist in a MATLAB structure?

MATLAB gives an error saying the field does not exist. You should check with isfield before accessing to avoid this.

How do you add a new field 'Height' with value 170 to an existing structure 'person'?

A person.Height = 170;

B addfield(person, 'Height', 170);

C person('Height') = 170;

D person->Height = 170;

In MATLAB, you add or modify fields using dot notation like person.Height = 170;.

Which function checks if a field exists in a structure?

A fieldexists()

B hasfield()

C isfield()

D checkfield()

The correct function is isfield(structure, 'fieldname').

What is the output of person.Name if person.Name = 'Bob';?

A Name

B Error

C person.Name

D 'Bob'

Accessing person.Name returns the string stored in that field, here 'Bob'.

How do you create an empty structure with fields 'X' and 'Y'?

A struct(X, Y)

B struct('X', [], 'Y', [])

C emptystruct('X', 'Y')

D newstruct('X', 'Y')

Use struct('X', [], 'Y', []) to create a structure with empty fields.

What happens if you try to access person.Weight but 'Weight' is not a field?

A Throws an error

B Returns empty

C Returns zero

D Creates the field automatically

MATLAB throws an error if you access a non-existent field.

Explain how to create a MATLAB structure and access its fields.

Think about how you name fields and get their values.

Describe how to safely check if a field exists before accessing it in a structure.

Use a function that returns true or false for field presence.

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