Structures and field access in MATLAB - Mini Project: Build & Apply

Structures and Field Access in MATLAB

📖 Scenario: You are working on a simple contact list application. Each contact has a name, phone number, and email address. You want to store this information using MATLAB structures.

🎯 Goal: Create a MATLAB structure to hold contact information, add a configuration variable to select a contact, access the fields of the selected contact, and display the contact details.

📋 What You'll Learn

Create a structure array called contacts with exactly three contacts.

Each contact must have fields name, phone, and email with exact values.

Create a variable called selectedIndex to choose which contact to display.

Use field access to get the selected contact's details.

Print the selected contact's name, phone, and email exactly as specified.

💡 Why This Matters

🌍 Real World

Structures are useful to organize related data like contact lists, product catalogs, or student records in MATLAB.

💼 Career

Understanding structures and field access is important for data organization and manipulation in engineering, data analysis, and scientific computing jobs.

1

Create the contacts structure array

Create a structure array called contacts with three contacts. The first contact has name 'Alice', phone '123-456-7890', and email '[e-mail]'. The second contact has name 'Bob', phone '234-567-8901', and email '[e-mail]'. The third contact has name 'Charlie', phone '345-678-9012', and email '[e-mail]'.

MATLAB

% Create the contacts structure array with three contacts
% Your code here

Hint

Use the syntax contacts(index).field = value; to create each contact.

2

Add a selectedIndex variable

Create a variable called selectedIndex and set it to 2 to select the second contact.

MATLAB

contacts(1).name = 'Alice';
contacts(1).phone = '123-456-7890';
contacts(1).email = '[e-mail]';
contacts(2).name = 'Bob';
contacts(2).phone = '234-567-8901';
contacts(2).email = '[e-mail]';
contacts(3).name = 'Charlie';
contacts(3).phone = '345-678-9012';
contacts(3).email = '[e-mail]';

% Create selectedIndex variable and set it to 2
% Your code here

Hint

Just write selectedIndex = 2; on a new line.

3

Access the selected contact's fields

Create three variables called selectedName, selectedPhone, and selectedEmail. Assign them the name, phone, and email fields of the contact at index selectedIndex in contacts.

MATLAB

contacts(1).name = 'Alice';
contacts(1).phone = '123-456-7890';
contacts(1).email = '[e-mail]';
contacts(2).name = 'Bob';
contacts(2).phone = '234-567-8901';
contacts(2).email = '[e-mail]';
contacts(3).name = 'Charlie';
contacts(3).phone = '345-678-9012';
contacts(3).email = '[e-mail]';

selectedIndex = 2;

% Assign selectedName, selectedPhone, and selectedEmail from contacts(selectedIndex)
% Your code here

Hint

Use dot notation with contacts(selectedIndex) to get each field.

4

Display the selected contact's details

Write three disp statements to print the selected contact's details exactly as follows:
Name: Bob
Phone: [phone]
Email: [e-mail]

MATLAB

contacts(1).name = 'Alice';
contacts(1).phone = '123-456-7890';
contacts(1).email = '[e-mail]';
contacts(2).name = 'Bob';
contacts(2).phone = '234-567-8901';
contacts(2).email = '[e-mail]';
contacts(3).name = 'Charlie';
contacts(3).phone = '345-678-9012';
contacts(3).email = '[e-mail]';

selectedIndex = 2;

selectedName = contacts(selectedIndex).name;
selectedPhone = contacts(selectedIndex).phone;
selectedEmail = contacts(selectedIndex).email;

% Display the selected contact's details using disp
% Your code here

Hint

Use disp(['Label: ' variable]) to print each line.

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