Matrix creation in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you create a 3x3 matrix with all elements equal to zero in MATLAB?

Use the zeros(3,3) function. It creates a 3-by-3 matrix filled with zeros.

beginner

What MATLAB function creates a 4x4 identity matrix?

Use eye(4). It creates a 4-by-4 matrix with ones on the diagonal and zeros elsewhere.

intermediate

How can you create a matrix with values from 1 to 9 arranged in 3 rows and 3 columns?

Use reshape(1:9, 3, 3). It reshapes the vector 1 to 9 into a 3x3 matrix.

beginner

What does the MATLAB command rand(2,3) do?

It creates a 2-by-3 matrix with random values between 0 and 1.

intermediate

How do you create a matrix filled with the number 7 of size 2x4 in MATLAB?

Use 7 * ones(2,4). The ones function creates a matrix of ones, then multiply by 7.

Which MATLAB function creates a matrix with all elements equal to one?

A ones()

B zeros()

C eye()

D rand()

The ones() function creates a matrix filled with ones.

What is the size of the matrix created by zeros(5,2)?

A 5 rows and 2 columns

B 2 rows and 5 columns

C 5 rows and 5 columns

D 2 rows and 2 columns

zeros(5,2) creates a matrix with 5 rows and 2 columns.

What does eye(3) produce?

A 3x3 matrix of ones

B 3x3 zero matrix

C 3x3 identity matrix

D 3x3 random matrix

eye(3) creates a 3x3 identity matrix with ones on the diagonal.

How do you create a 2x3 matrix with random numbers between 0 and 1?

A ones(2,3)

B rand(2,3)

C zeros(2,3)

D eye(2,3)

rand(2,3) creates a 2x3 matrix with random values between 0 and 1.

Which command reshapes a vector into a matrix?

A rand()

B zeros()

C eye()

D reshape()

The reshape() function changes the shape of a vector or matrix.

Explain how to create a matrix of any size filled with a specific number in MATLAB.

Think about starting with a matrix of ones.

Describe the difference between zeros(), ones(), and eye() functions in MATLAB.

Focus on what each function fills the matrix with.

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