Matrix creation in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a 3x3 matrix of zeros.

MATLAB

A = [1](3,3);

Drag options to blanks, or click blank then click option'

Aeye

Bones

Czeros

Drand

Attempts:

3 left

💡 Hint

Common Mistakes

Using ones instead of zeros creates a matrix of ones.

Using eye creates an identity matrix, not zeros.

The zeros function creates a matrix filled with zeros. Here, zeros(3,3) creates a 3x3 matrix of zeros.

2fill in blank

medium

Complete the code to create a 4x4 identity matrix.

MATLAB

I = [1](4);

Drag options to blanks, or click blank then click option'

Azeros

Beye

Cones

Drand

Attempts:

3 left

💡 Hint

Common Mistakes

Using zeros creates a matrix of zeros, not identity.

Using ones creates a matrix of ones, not identity.

The eye function creates an identity matrix. eye(4) creates a 4x4 identity matrix.

3fill in blank

hard

Fix the error in the code to create a 2x5 matrix filled with ones.

MATLAB

M = [1](2, 5);

Drag options to blanks, or click blank then click option'

Azeros

Beye

Crand

Dones

Attempts:

3 left

💡 Hint

Common Mistakes

Using zeros creates a matrix of zeros.

Using eye creates an identity matrix, which is square.

The ones function creates a matrix filled with ones. ones(2,5) creates a 2x5 matrix of ones.

4fill in blank

hard

Fill both blanks to create a 3x3 matrix with random values between 0 and 1.

MATLAB

R = [1]([2], 3);

Drag options to blanks, or click blank then click option'

Arand

B3

C4

Drandn

Attempts:

3 left

💡 Hint

Common Mistakes

Using randn generates random numbers with a normal distribution.

Using wrong size arguments causes dimension errors.

The rand function creates a matrix with random values between 0 and 1. rand(3,3) creates a 3x3 matrix.

5fill in blank

hard

Fill both blanks to create a 2x4 matrix of twos using element-wise multiplication.

MATLAB

M = [1](2, 4) [2] 2;

Drag options to blanks, or click blank then click option'

Aones

B*

D.*

Attempts:

3 left

💡 Hint

Common Mistakes

Using regular multiplication * instead of element-wise .* causes errors.

Leaving the last blank with an operator causes syntax errors.

ones(2,4) creates a matrix of ones. Using element-wise multiplication .* with 2 multiplies each element by 2. The last blank is empty because no extra operator is needed.

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