Logical indexing in MATLAB - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is logical indexing in MATLAB?

Logical indexing is a way to select elements from an array using a logical array of the same size, where true means select the element and false means skip it.

beginner

How do you create a logical index to select elements greater than 5 from array A?

Use the expression idx = A > 5; which returns a logical array where elements of A greater than 5 are true.

beginner

Given A = [2, 7, 4, 9], what does A(A > 5) return?

It returns [7 9] because only elements greater than 5 are selected using logical indexing.

intermediate

Can logical indexing be used to modify elements in an array? How?

Yes. You can assign new values to elements selected by logical indexing. For example, A(A < 3) = 0; sets all elements less than 3 in A to zero.

intermediate

What happens if the logical index array is not the same size as the original array?

MATLAB will give an error because logical indexing requires the logical array to be the same size as the array being indexed.

What does the expression B = A(A > 10); do in MATLAB?

A Sets elements of A greater than 10 to zero

B Returns a logical array of elements greater than 10

C Selects elements of A greater than 10 and stores them in B

D Generates an error

The expression uses logical indexing to select elements of A that are greater than 10 and stores them in B.

Which of these is a valid logical index for array A?

A [true false true]

B [1 2 3]

C [0 1 2]

D [5 6 7]

Logical indices must be logical arrays (true/false). Numeric arrays like [1 2 3] are not valid logical indices.

What will happen if you try A([true false]) when A has 3 elements?

A Returns all elements

B Error due to size mismatch

C Returns first two elements

D Returns first element only

Logical index arrays must be the same size as the array being indexed, otherwise MATLAB throws an error.

How can you change all negative values in array A to zero using logical indexing?

A A(A < 0) = 0;

B A = 0 where A < 0;

C A = A > 0;

D A = zeros(size(A));

Using logical indexing, you select elements less than zero and assign zero to them.

If idx = A > 3;, what type is idx?

A String array

B Numeric array

C Cell array

D Logical array

The result of a comparison like A > 3 is a logical array of true/false values.

Explain how logical indexing works in MATLAB and give an example of selecting elements from an array.

Think about how you can use true/false arrays to pick elements.

Describe how to modify elements of an array using logical indexing in MATLAB.

Consider how you can assign new values to parts of an array.

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