Matrix indexing (row, col) in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does matrix indexing with (row, col) mean in MATLAB?

It means selecting an element from a matrix by specifying its row number and column number, like A(row, col).

beginner

How do you access the element in the 2nd row and 3rd column of matrix A?

Use A(2, 3) to get the element at row 2, column 3.

beginner

What happens if you use a colon ':' in matrix indexing like A(1, :)?

The colon ':' selects all columns in the specified row. So A(1, :) returns all elements in the first row.

beginner

How can you select an entire column from a matrix?

Use A(:, col) where col is the column number. This selects all rows in that column.

intermediate

Can you use logical indexing with matrix (row, col) indexing?

Yes, you can use logical arrays to select elements, but logical indexing usually works with linear indexing or logical masks.

What does A(3, 2) return in MATLAB?

A Element in 3rd row, 2nd column of A

B Element in 2nd row, 3rd column of A

C All elements in 3rd row

D All elements in 2nd column

A(row, col) accesses the element at the specified row and column.

How do you select all rows in the 4th column of matrix A?

A A(4)

B A(4, :)

C A(:, 4)

D A(:, :)

A(:, 4) selects all rows in column 4.

What does the colon ':' operator do in matrix indexing?

A Selects the first element only

B Selects all elements in that dimension

C Deletes the selected elements

D Creates a new matrix

The colon ':' selects all elements along the specified dimension.

If A is a 5x5 matrix, what does A(2:4, 3) return?

A Elements from rows 2 to 4 in column 3

B Elements from columns 2 to 4 in row 3

C All elements in row 3

D All elements in column 2

A(2:4, 3) selects rows 2 to 4 in column 3.

Can you use negative indices in MATLAB matrix indexing?

A Yes, to count from the end

B Only for rows

C Only for columns

D No, MATLAB does not support negative indices

MATLAB does not support negative indices; use 'end' keyword instead.

Explain how to select a single element and an entire row from a matrix using (row, col) indexing in MATLAB.

Think about how you specify row and column numbers and how ':' works.

Describe how to select multiple rows and a single column from a matrix in MATLAB.

Use row range like 2:4 and specify the column number.

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