Matrix indexing (row, col) in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to access the element in the 2nd row and 3rd column of matrix A.

MATLAB

element = A([1], 3);

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

A2

B4

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using the column number instead of the row number.

Confusing row and column order.

In MATLAB, the first index is the row number. To access the 2nd row, use 2.

2fill in blank

medium

Complete the code to extract the entire 3rd column from matrix B.

MATLAB

col3 = B(:, [1]);

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A4

B2

C3

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using ':' for columns instead of rows.

Selecting the wrong column number.

The colon ':' means all rows, and the column index is 3 to get the 3rd column.

3fill in blank

hard

Fix the error in the code to assign 10 to the element in the 1st row and 4th column of matrix C.

MATLAB

C([1], 4) = 10;

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A4

B1

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as an index, which is invalid in MATLAB.

Confusing row and column indices.

Matrix indices in MATLAB start at 1, so the 1st row is indexed by 1.

4fill in blank

hard

Fill both blanks to create a submatrix from matrix D containing rows 2 to 4 and columns 1 to 3.

MATLAB

submatrix = D([1], [2]);

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A2:4

B1:2

C1:3

D3:5

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect ranges that don't match the desired rows or columns.

Swapping row and column ranges.

Use '2:4' to select rows 2 to 4 and '1:3' to select columns 1 to 3.

5fill in blank

hard

Fill all three blanks to create a matrix E where each element is the sum of the row and column indices from matrix F.

MATLAB

E = zeros(3, 3);
for [1] = 1:3
    for [2] = 1:3
        E([1], [2]) = [1] + [2];
    end
end

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

Ai

Bj

Dk

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same variable name for both loops.

Mixing up row and column indices in assignment.

Use 'i' for the outer loop (rows), 'j' for the inner loop (columns), and index E(i, j) to assign the sum.

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