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.