Logical indexing in MATLAB - Mini Project: Build & Apply

Logical Indexing in MATLAB

📖 Scenario: You work in a small store and have a list of product prices. You want to find which products are expensive, meaning their price is above a certain amount.

🎯 Goal: Learn how to use logical indexing in MATLAB to select and display only the expensive products from a list of prices.

📋 What You'll Learn

Create a vector of product prices

Create a threshold price variable

Use logical indexing to find prices above the threshold

Display the filtered expensive prices

💡 Why This Matters

🌍 Real World

Stores often need to filter products by price to create sales or promotions.

💼 Career

Logical indexing is a key skill in data analysis and helps quickly select data based on conditions.

1

Create the product prices vector

Create a vector called prices with these exact values: [10, 25, 30, 15, 50, 5]

MATLAB

% Create a vector called prices with the values [10, 25, 30, 15, 50, 5]
% Your code here

Hint

Use square brackets to create a vector in MATLAB.

2

Set the threshold price

Create a variable called threshold and set it to 20

MATLAB

prices = [10, 25, 30, 15, 50, 5];
% Create a variable called threshold and set it to 20
% Your code here

Hint

Just assign the number 20 to the variable threshold.

3

Use logical indexing to find expensive products

Create a variable called expensive that uses logical indexing on prices to select only prices greater than threshold

MATLAB

prices = [10, 25, 30, 15, 50, 5];
threshold = 20;
% Use logical indexing to create expensive with prices greater than threshold
% Your code here

Hint

Use prices > threshold inside parentheses to select elements.

4

Display the expensive products

Write a disp statement to display the variable expensive

MATLAB

prices = [10, 25, 30, 15, 50, 5];
threshold = 20;
expensive = prices(prices > threshold);
% Display the expensive prices
% Your code here

Hint

Use disp(expensive) to show the filtered prices.

Practice

1. What does logical indexing in MATLAB allow you to do with an array A and a logical array L of the same size?

easy

A. Sort the elements of A based on L

B. Select elements of A where L is true

C. Change the size of A to match L

D. Convert A into a logical array

Solution

1. Step 1: Understand logical indexing concept

   Logical indexing uses a logical array to pick elements from another array where the logical array is true.

2. Step 2: Apply to arrays A and L

   Elements of A are selected only where L has true values, filtering A.

3. Final Answer:

   Select elements of A where L is true -> Option B

4. Quick Check:

   Logical indexing = select elements [OK]

Hint: Logical array true picks elements from original array [OK]

Common Mistakes:

• Thinking logical indexing changes array size arbitrarily
• Confusing logical indexing with sorting
• Assuming logical indexing converts data types

2. Which of the following is the correct syntax to select elements of vector v that are greater than 5 using logical indexing?

easy

A. v(v > 5)

B. v > 5(v)

C. v{v > 5}

D. v[v > 5]

Solution

1. Step 1: Identify correct logical indexing syntax

   In MATLAB, logical indexing uses parentheses with a logical condition inside, like v(condition).

2. Step 2: Check each option

   v(v > 5) uses v(v > 5), which is correct syntax. The other options use invalid indexing forms.

3. Final Answer:

   v(v > 5) -> Option A

4. Quick Check:

   Logical indexing syntax = v(condition) [OK]

Hint: Use parentheses with condition inside for logical indexing [OK]

Common Mistakes:

• Using curly braces or square brackets for logical indexing
• Placing condition outside parentheses
• Confusing logical indexing with cell or array indexing

3. Given A = [3, 7, 2, 9, 5], what is the result of A(A > 4)?

medium

A. [2, 5]

B. [3, 7, 2, 9, 5]

C. [7, 9]

D. [7, 9, 5]

Solution

1. Step 1: Create logical array for condition A > 4

   Check each element: 3>4?false, 7>4?true, 2>4?false, 9>4?true, 5>4?true. Logical array = [false, true, false, true, true].

2. Step 2: Select elements of A where logical array is true

   Elements at true positions: 7, 9, 5.

3. Final Answer:

   [7, 9, 5] -> Option D

4. Quick Check:

   Elements > 4 = [7, 9, 5] [OK]

Hint: Filter elements by condition, then pick true positions [OK]

Common Mistakes:

• Including elements not greater than 4
• Confusing logical indexing with logical AND operation
• Returning original array instead of filtered

4. What is wrong with this MATLAB code snippet?

A = [1, 4, 6, 8];
L = [true, false];
B = A(L);

medium

A. Logical array L size does not match A size

B. Logical array L must contain only numeric values

C. Cannot use logical indexing on row vectors

D. Syntax error in logical indexing

Solution

1. Step 1: Check sizes of A and L

   A has 4 elements, L has 2 elements. Logical indexing requires L to be same size as A.

2. Step 2: Identify error cause

   Since sizes differ, MATLAB will give an error when using L to index A.

3. Final Answer:

   Logical array L size does not match A size -> Option A

4. Quick Check:

   Logical array size must match data size [OK]

Hint: Logical array must be same size as indexed array [OK]

Common Mistakes:

• Assuming logical array can be smaller
• Thinking logical array must be numeric
• Believing logical indexing syntax is wrong here

5. You have a matrix M = [2, 5, 8; 1, 7, 3; 4, 6, 9]. How do you replace all elements less than 5 with zero using logical indexing?

hard

A. M(M <= 5) = 0;

B. M(M > 5) = 0;

C. M(M < 5) = 0;

D. M(M == 5) = 0;

Solution

1. Step 1: Identify elements less than 5 in M

   Elements less than 5 are 2, 1, 3, 4.

2. Step 2: Use logical indexing to assign zero to these elements

   Using M(M < 5) = 0; replaces all elements less than 5 with zero.

3. Final Answer:

   M(M < 5) = 0; -> Option C

4. Quick Check:

   Replace elements < 5 with 0 using M(M < 5) = 0 [OK]

Hint: Use condition inside parentheses to assign new values [OK]

Common Mistakes:

• Replacing wrong elements by using > or == instead of <
• Using incorrect logical operators
• Trying to assign without logical indexing