Logical indexing in MATLAB - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of logical indexing with condition

What is the output of the following MATLAB code?

MATLAB

A = [10, 20, 30, 40, 50];
B = A(A > 25);

A [10 20 30]

B [30 40 50]

C [25 30 40 50]

D [10 20 40 50]

💡 Hint

Logical indexing selects elements where the condition is true.

The condition A > 25 is true for elements 30, 40, and 50. So B contains these values.

❓ Predict Output

intermediate

Logical indexing with multiple conditions

What does the variable C contain after running this code?

MATLAB

A = [5, 12, 7, 20, 3];
C = A(A > 5 & A < 20);

A [12 7 20]

B [12 20]

C [12 7]

D [5 12 7]

💡 Hint

Use & to combine conditions and select elements between 5 and 20, excluding 5 and 20.

Only 12 and 7 satisfy both conditions: greater than 5 and less than 20.

❓ Predict Output

advanced

Logical indexing with matrix and assignment

What is the value of matrix M after executing this code?

MATLAB

M = [1 2 3; 4 5 6; 7 8 9];
M(M > 4) = 0;

A [1 2 3; 4 0 0; 0 0 0]

B [1 2 3; 4 5 6; 7 8 0]

C [1 2 3; 0 0 0; 0 0 0]

D [0 0 0; 4 5 6; 7 8 9]

💡 Hint

Elements greater than 4 are replaced by zero.

All elements greater than 4 are set to zero, so 5,6,7,8,9 become 0.

❓ Predict Output

advanced

Logical indexing with empty result

What is the output of this code snippet?

MATLAB

A = [2, 4, 6, 8];
B = A(A < 0);

A Error: Index exceeds matrix dimensions.

B [0]

C [2 4 6 8]

D []

💡 Hint

No elements satisfy the condition A < 0.

Since no elements are less than zero, the result is an empty array.

🧠 Conceptual

expert

Logical indexing behavior with NaN values

Given the vector V = [1, NaN, 3, NaN, 5], what is the result of V(~isnan(V))?

A [1 3 5]

B [NaN NaN]

C [1 NaN 3 NaN 5]

D Error: Invalid logical indexing

💡 Hint

The ~ operator negates the logical array returned by isnan.

isnan(V) returns true for NaN elements; ~isnan(V) is true for non-NaN elements, so only 1,3,5 are selected.

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