Logical indexing in MATLAB - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to select elements greater than 5 from the array.

MATLAB

A = [2, 7, 4, 9, 1];
result = A(A [1] 5);

Drag options to blanks, or click blank then click option'

A>

B<

C==

D~=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' instead of '>' will select elements less than 5.

Using '==' will select only elements equal to 5.

The operator '>' selects elements greater than 5 using logical indexing.

2fill in blank

medium

Complete the code to replace elements less than 3 with zero using logical indexing.

MATLAB

A = [1, 4, 2, 6, 3];
A(A [1] 3) = 0;

Drag options to blanks, or click blank then click option'

A<=

B>

C<

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' will select elements greater than 3, which is incorrect here.

Using '<=' will include elements equal to 3, which is not intended.

The operator '<' selects elements less than 3 to replace them with zero.

3fill in blank

hard

Fix the error in the code to select even numbers using logical indexing.

MATLAB

A = [1, 2, 3, 4, 5];
evenNums = A(mod(A, 2) [1] 0);

Drag options to blanks, or click blank then click option'

A==

B<

C>

D~=

Attempts:

3 left

💡 Hint

Common Mistakes

Using '~=' selects odd numbers instead of even.

Using '>' or '<' does not correctly identify even numbers.

The '==' operator checks where the remainder is zero, selecting even numbers.

4fill in blank

hard

Fill both blanks to create a logical index for elements between 3 and 7 (inclusive).

MATLAB

A = [1, 4, 6, 8, 3];
idx = (A [1] 3) & (A [2] 7);

Drag options to blanks, or click blank then click option'

A>=

B<

C<=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' excludes 3, which is incorrect.

Using '<' excludes 7, which is incorrect.

Use '>=' for elements greater or equal to 3 and '<=' for elements less or equal to 7.

5fill in blank

hard

Fill all three blanks to create a dictionary of elements and their squares for elements greater than 2.

MATLAB

A = [1, 3, 5, 2];
result = containers.Map(A(A [1] 2), A(A [2] 2) [3] 2);

Drag options to blanks, or click blank then click option'

A>

B.*

C.^

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using '.*' multiplies by 2 instead of squaring.

Using '+' adds 2 instead of squaring.

Select elements greater than 2 and map them to their squares using '.^ 2'.

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