Matrix indexing (row, col) in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this matrix indexing code?

Consider the matrix A = [1 2 3; 4 5 6; 7 8 9]; What is the value of A(2,3)?

MATLAB

A = [1 2 3; 4 5 6; 7 8 9];
result = A(2,3);

A 3

B 6

C 8

D 5

💡 Hint

Remember that the first index is the row number and the second is the column number.

In MATLAB, A(2,3) means row 2, column 3 of matrix A. The element there is 6.

❓ Predict Output

intermediate

What does this code output when extracting a submatrix?

Given B = [10 20 30; 40 50 60; 70 80 90]; what is the output of B(1:2, 2:3)?

MATLAB

B = [10 20 30; 40 50 60; 70 80 90];
sub = B(1:2, 2:3);

A [50 60; 80 90]

B [10 20; 40 50]

C [20 30; 50 60]

D [30 60; 60 90]

💡 Hint

The colon operator selects ranges of rows and columns.

Rows 1 and 2, columns 2 and 3 of B are selected, resulting in the 2x2 matrix [20 30; 50 60].

❓ Predict Output

advanced

What is the output of this linear indexing code?

Given C = [3 6 9; 12 15 18; 21 24 27]; what is the value of C(5)?

MATLAB

C = [3 6 9; 12 15 18; 21 24 27];
val = C(5);

A 12

B 18

C 24

D 15

💡 Hint

MATLAB stores matrices column-wise when using single index.

Linear indexing counts down columns first. The 5th element is in column 2, row 2, which is 15.

❓ Predict Output

advanced

What error does this code produce?

What happens when you run this code?
D = [1 2; 3 4]; val = D(3,3);

MATLAB

D = [1 2; 3 4];
val = D(3,3);

A Index exceeds matrix dimensions.

B val = 3

C val = 4

D val = 0

💡 Hint

Check the size of matrix D and the indices used.

Matrix D is 2x2, so index (3,3) is outside its size, causing an indexing error.

🧠 Conceptual

expert

How many elements are in the matrix after this indexing operation?

Given E = magic(4); what is the number of elements in F = E(2:4, 1:3);?

MATLAB

E = magic(4);
F = E(2:4, 1:3);
num_elements = numel(F);

A 9

B 12

C 7

D 6

💡 Hint

Count the rows and columns selected and multiply.

Rows 2 to 4 means 3 rows, columns 1 to 3 means 3 columns, so 3*3=9 elements.

Practice

1. In MATLAB, how do you access the element in the 2nd row and 3rd column of a matrix A?

easy

A. A(3, 2)

B. A[2, 3]

C. A(2, 3)

D. A{2, 3}

Solution

1. Step 1: Understand MATLAB indexing syntax

   MATLAB uses parentheses with row and column indices in the form A(row, column).

2. Step 2: Apply to given indices

   To get the element in the 2nd row and 3rd column, use A(2, 3).

3. Final Answer:

   A(2, 3) -> Option C

4. Quick Check:

   Matrix indexing = A(2, 3) [OK]

Hint: Remember: first index is row, second is column [OK]

Common Mistakes:

• Swapping row and column indices
• Using square brackets instead of parentheses
• Using curly braces which are for cell arrays

2. Which of the following is the correct syntax to select the entire 4th row of a matrix B in MATLAB?

easy

A. B(:, 4)

B. B(4, :)

C. B(4)

D. B[4, :]

Solution

1. Step 1: Recall how to select entire rows or columns

   In MATLAB, : means all elements in that dimension. To select the entire 4th row, use B(4, :).

2. Step 2: Check options

   B(:, 4) selects the 4th column, B(4) is invalid for 2D matrix, and square brackets are not used for indexing.

3. Final Answer:

   B(4, :) -> Option B

4. Quick Check:

   Row selection = B(4, :) [OK]

Hint: Use ':' for all columns in a specific row [OK]

Common Mistakes:

• Confusing row and column positions
• Using square brackets for indexing
• Omitting the colon for full dimension selection

3. Given the matrix C = [1 2 3; 4 5 6; 7 8 9], what is the output of C(2, 1:3)?

medium

A. [4 5 6]

B. [1 2 3]

C. [7 8 9]

D. [5 6 7]

Solution

1. Step 1: Identify the row and columns selected

   C(2, 1:3) means row 2, columns 1 through 3.

2. Step 2: Extract elements from matrix

   Row 2 of C is [4 5 6]. Selecting columns 1 to 3 gives the entire row.

3. Final Answer:

   [4 5 6] -> Option A

4. Quick Check:

   Row 2, cols 1-3 = [4 5 6] [OK]

Hint: Use row index first, then column range with ':' [OK]

Common Mistakes:

• Selecting wrong row or column range
• Confusing row and column indices
• Expecting a single number instead of a vector

4. What is wrong with this MATLAB code snippet if D = [10 20 30; 40 50 60] and we want to select the 3rd row?
row3 = D(3, :);

medium

A. Syntax error due to wrong indexing

B. Returns an empty array

C. Returns the last row instead of the 3rd

D. Runtime error because row 3 does not exist

Solution

1. Step 1: Check matrix size

   Matrix D has 2 rows and 3 columns only.

2. Step 2: Analyze indexing for row 3

   Trying to access row 3 with D(3, :) is invalid and causes a runtime error because that row does not exist.

3. Final Answer:

   Runtime error because row 3 does not exist -> Option D

4. Quick Check:

   Index out of bounds = runtime error [OK]

Hint: Check matrix size before indexing rows or columns [OK]

Common Mistakes:

• Assuming MATLAB returns empty for out-of-range index
• Confusing syntax error with runtime error
• Expecting MATLAB to auto-expand matrix

5. You have a matrix E = [5 10 15; 20 25 30; 35 40 45]. How do you create a new matrix F that contains only the 1st and 3rd rows and the 2nd and 3rd columns of E?

hard

A. F = E([1 3], [2 3]);

B. F = E(1:3, 2:3);

C. F = E(:, [1 3]);

D. F = E([2 3], [1 2]);

Solution

1. Step 1: Identify required rows and columns

   We want rows 1 and 3, and columns 2 and 3 from matrix E.

2. Step 2: Use indexing with vectors for rows and columns

   In MATLAB, use E([1 3], [2 3]) to select these specific rows and columns.

3. Final Answer:

   F = E([1 3], [2 3]); -> Option A

4. Quick Check:

   Index rows and columns with vectors = F = E([1 3], [2 3]) [OK]

Hint: Use vectors in parentheses to pick specific rows and columns [OK]

Common Mistakes:

• Using colon to select unwanted rows or columns
• Swapping row and column indices
• Using incorrect row or column numbers