Boolean indexing in NumPy - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Boolean Indexing on a NumPy Array

What is the output of this code snippet using Boolean indexing on a NumPy array?

NumPy

import numpy as np
arr = np.array([10, 15, 20, 25, 30])
result = arr[arr > 20]
print(result)

A [15 20 25]

B [25 30]

C [10 15 20]

D [20 25 30]

💡 Hint

Remember, Boolean indexing selects elements where the condition is True.

The condition arr > 20 selects elements greater than 20, which are 25 and 30.

❓ data_output

intermediate

Number of Elements Selected by Boolean Indexing

How many elements are selected by the Boolean indexing in this code?

NumPy

import numpy as np
arr = np.array([5, 12, 17, 9, 3, 21])
selected = arr[(arr >= 10) & (arr < 20)]
print(len(selected))

A 5

B 3

C 4

D 2

💡 Hint

Count elements between 10 (inclusive) and 20 (exclusive).

Elements satisfying (arr >= 10) & (arr < 20): 12 and 17 (9 < 10 excluded, 21 >= 20 excluded), so 2 elements.

❓ Predict Output

advanced

Output of Boolean Indexing with Multiple Conditions

What is the output of this code using Boolean indexing with multiple conditions?

NumPy

import numpy as np
arr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])
result = arr[(arr % 2 == 0) | (arr > 7)]
print(result)

A [2 4 6 8 9]

B [8 9]

C [2 4 6 8]

D [1 3 5 7]

💡 Hint

Select elements that are even or greater than 7.

Elements that are even: 2,4,6,8; elements greater than 7: 8,9; combined unique elements: 2,4,6,8,9.

🔧 Debug

advanced

Identify the Error in Boolean Indexing Code

What error does this code raise when using Boolean indexing?

NumPy

import numpy as np
arr = np.array([1, 2, 3, 4, 5])
mask = arr > 3
result = arr[mask == True]
print(result)

A No error, output: [4 5]

B IndexError

C TypeError

D ValueError

💡 Hint

Check if comparing Boolean array to True affects indexing.

mask == True returns the same Boolean array, so indexing works and outputs [4 5].

🚀 Application

expert

Filter Rows in 2D Array Using Boolean Indexing

Given this 2D NumPy array, which option correctly selects rows where the second column is less than 5?

NumPy

import numpy as np
arr = np.array([[1, 4, 7], [2, 5, 8], [3, 3, 9], [4, 6, 10]])
result = ???
print(result)

A arr[:, arr[1, :] < 5]

B arr[arr[1, :] < 5]

C arr[arr[:, 1] < 5]

D arr[arr[:, 1] > 5]

💡 Hint

Use Boolean indexing on rows based on the second column values.

arr[:, 1] selects the second column; arr[:, 1] < 5 creates a Boolean mask for rows; indexing arr with this mask selects rows where second column < 5.