Matrix creation in MATLAB - Mini Project: Build & Apply

Matrix creation

📖 Scenario: You are working with data in a lab where you need to organize numbers in a grid format. This grid is called a matrix. Matrices are useful for storing and working with numbers in rows and columns, like a table.

🎯 Goal: You will create a matrix in MATLAB with specific numbers arranged in rows and columns. This will help you understand how to build and display matrices.

📋 What You'll Learn

Create a matrix variable with exact numbers and arrangement

Create a variable for the number of rows in the matrix

Use a loop to calculate the sum of all elements in the matrix

Display the sum of the matrix elements

💡 Why This Matters

🌍 Real World

Matrices are used in many fields like engineering, physics, and computer graphics to organize and process data in rows and columns.

💼 Career

Knowing how to create and manipulate matrices is important for jobs in data analysis, scientific computing, and software development.

1

Create the matrix

Create a matrix called A with these exact values arranged in 3 rows and 3 columns:
1 2 3
4 5 6
7 8 9

MATLAB

% Create the matrix A with the given values
% Your code here

Hint

Use square brackets and semicolons to separate rows in MATLAB.

2

Create a variable for number of rows

Create a variable called rows that stores the number of rows in the matrix A using the size function.

MATLAB

A = [1 2 3; 4 5 6; 7 8 9];
% Create variable rows to hold number of rows in A
% Your code here

Hint

Use size(A, 1) to get the number of rows.

3

Calculate sum of all elements

Create a variable called totalSum and use a for loop with variable i from 1 to rows to add all elements of each row in A to totalSum. Initialize totalSum to 0 before the loop.

MATLAB

A = [1 2 3; 4 5 6; 7 8 9];
rows = size(A, 1);
% Initialize totalSum to 0
% Use for loop with i from 1 to rows to add sum of each row to totalSum
% Your code here

Hint

Use sum(A(i, :)) to add all elements in row i.

4

Display the total sum

Use disp to display the value of totalSum.

MATLAB

A = [1 2 3; 4 5 6; 7 8 9];
rows = size(A, 1);
totalSum = 0;
for i = 1:rows
    totalSum = totalSum + sum(A(i, :));
end
% Display totalSum
% Your code here

Hint

Use disp(totalSum) to show the sum.

Practice

1. Which of the following correctly creates a 2x3 matrix in MATLAB?

easy

A. [1 2; 3 4 5]

B. [1, 2; 3, 4; 5, 6]

C. [1 2 3; 4 5 6]

D. [1; 2; 3; 4]

Solution

1. Step 1: Understand matrix dimensions

   A 2x3 matrix has 2 rows and 3 columns.

2. Step 2: Check each option's shape

   [1 2 3; 4 5 6] has 2 rows separated by semicolon and 3 columns separated by spaces, so it is 2x3.

3. Final Answer:

   [1 2 3; 4 5 6] -> Option C

4. Quick Check:

   2 rows and 3 columns = [1 2 3; 4 5 6] [OK]

Hint: Rows separated by semicolons, columns by spaces or commas [OK]

Common Mistakes:

• Using commas instead of semicolons for rows
• Mismatched number of columns in rows
• Confusing rows and columns

2. Which MATLAB code correctly creates a 3x3 identity matrix?

easy

A. eye(3)

B. zeros(3,3)

C. ones(3)

D. diag(3)

Solution

1. Step 1: Recall identity matrix function

   In MATLAB, eye(n) creates an n-by-n identity matrix.

2. Step 2: Check options

   eye(3) uses eye(3), which is the correct function for a 3x3 identity matrix.

3. Final Answer:

   eye(3) -> Option A

4. Quick Check:

   Identity matrix = eye(3) [OK]

Hint: Use eye(n) for identity matrix of size n [OK]

Common Mistakes:

• Using zeros or ones instead of eye
• Using diag with incorrect vector size
• Confusing identity with zero matrix

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

A = [1 2; 3 4];
B = zeros(2,3);
C = [A B];
disp(C)

medium

A. [1 2; 3 4; 0 0 0]

B. [1 2 0 0 0; 3 4 0 0 0]

C. [1 2; 3 4; 0 0]

D. Error: Matrix dimensions must agree.

Solution

1. Step 1: Understand matrix sizes

   A is 2x2, B is 2x3 of zeros.

2. Step 2: Concatenate horizontally

   [A B] joins matrices side by side, resulting in 2 rows and 5 columns.

3. Final Answer:

   [1 2 0 0 0; 3 4 0 0 0] -> Option B

4. Quick Check:

   Horizontal concat of 2x2 and 2x3 = 2x5 matrix [OK]

Hint: Horizontal concat needs same row count [OK]

Common Mistakes:

• Trying to concat matrices with different row counts vertically
• Confusing zeros size
• Expecting error instead of valid concat

4. Identify the error in this MATLAB code:

M = [1 2 3; 4 5];

medium

A. Missing semicolon between rows

B. Matrix elements must be separated by commas

C. Use parentheses instead of square brackets

D. Rows have different number of columns

Solution

1. Step 1: Check matrix row sizes

   First row has 3 elements, second row has 2 elements.

2. Step 2: Understand MATLAB matrix rules

   All rows must have the same number of columns; here they differ.

3. Final Answer:

   Rows have different number of columns -> Option D

4. Quick Check:

   Matrix rows must match column count [OK]

Hint: All rows must have equal columns in matrix [OK]

Common Mistakes:

• Thinking commas are required between rows
• Using parentheses instead of brackets
• Ignoring unequal row lengths

5. You want to create a 4x4 matrix with 1s on the diagonal and 0.5 everywhere else. Which MATLAB code achieves this?

hard

A. M = 0.5*ones(4); M(eye(4) == 1) = 1;

B. M = eye(4) + 0.5;

C. M = 0.5*ones(4) + eye(4);

D. M = ones(4); M(eye(4) == 1) = 0.5;

Solution

1. Step 1: Create a 4x4 matrix of 0.5

   Using 0.5*ones(4) sets all elements to 0.5.

2. Step 2: Replace diagonal elements with 1

   Using logical indexing M(eye(4) == 1) = 1; sets diagonal to 1.

3. Final Answer:

   M = 0.5*ones(4); M(eye(4) == 1) = 1; -> Option A

4. Quick Check:

   Diagonal 1, others 0.5 = M = 0.5*ones(4); M(eye(4) == 1) = 1; [OK]

Hint: Set all to 0.5 then replace diagonal with 1 [OK]

Common Mistakes:

• Adding eye(4) instead of replacing diagonal
• Mixing addition and multiplication incorrectly
• Setting diagonal to 0.5 instead of 1