Matrix indexing (row, col) in MATLAB - Mini Project: Build & Apply

Matrix indexing (row, col)

📖 Scenario: You are working with a small grid of numbers representing temperatures recorded in a 3x3 area. You want to access specific temperature values by their row and column positions.

🎯 Goal: Learn how to create a matrix and access its elements using row and column indices in MATLAB.

📋 What You'll Learn

Create a 3x3 matrix with exact values

Define row and column index variables

Use matrix indexing with row and column to get a specific value

Display the accessed value

💡 Why This Matters

🌍 Real World

Matrix indexing is used in many fields like image processing, data analysis, and scientific computing to access and manipulate data stored in grids or tables.

💼 Career

Knowing how to index matrices is essential for engineers, data scientists, and researchers who work with numerical data and MATLAB programming.

1

Create the temperature matrix

Create a 3x3 matrix called temps with these exact values: first row [15, 18, 21], second row [16, 19, 22], third row [17, 20, 23].

MATLAB

% Your code here

Hint

Use square brackets and semicolons to separate rows in MATLAB.

2

Set row and column indices

Create two variables: row and col. Set row to 2 and col to 3.

MATLAB

temps = [15, 18, 21; 16, 19, 22; 17, 20, 23];
% Your code here

Hint

Assign the numbers 2 and 3 to variables row and col respectively.

3

Access the matrix element using row and column

Create a variable called value and set it to the element in temps at the position given by row and col using matrix indexing.

MATLAB

temps = [15, 18, 21; 16, 19, 22; 17, 20, 23];
row = 2;
col = 3;
% Your code here

Hint

Use parentheses with row and col inside to get the matrix element.

4

Display the accessed value

Write a command to display the value stored in value.

MATLAB

temps = [15, 18, 21; 16, 19, 22; 17, 20, 23];
row = 2;
col = 3;
value = temps(row, col);
% Your code here

Hint

Use disp(value) to show the number on the screen.

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