Creating boolean arrays in NumPy - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of boolean array creation with comparison

What is the output of this code snippet that creates a boolean array by comparing elements of a numpy array to a value?

NumPy

import numpy as np
arr = np.array([3, 7, 1, 5])
result = arr > 4
print(result)

A [True False True False]

B [False True False True]

C [False False False False]

D [True True True True]

💡 Hint

Think about which numbers in the array are greater than 4.

The code compares each element of the array to 4. Elements 7 and 5 are greater than 4, so their positions are True; others are False.

❓ data_output

intermediate

Count of True values in a boolean array

Given a boolean array created by comparing elements of a numpy array, how many True values does it contain?

NumPy

import numpy as np
arr = np.array([10, 15, 20, 5, 0])
bool_arr = arr <= 10
count_true = np.sum(bool_arr)
print(count_true)

A 4

B 2

C 1

D 3

💡 Hint

Count how many elements are less than or equal to 10.

Elements 10, 5, and 0 satisfy the condition, so there are 3 True values.

🔧 Debug

advanced

Identify the error in boolean array creation

What error does this code raise when trying to create a boolean array by comparing elements of a numpy array?

NumPy

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

A SyntaxError

B TypeError

C NameError

D IndexError

💡 Hint

Look carefully at the comparison operator usage.

The comparison operator '>' is incomplete without a value to compare to, causing a SyntaxError.

🚀 Application

advanced

Filter array elements using a boolean array

Which option correctly filters elements greater than 50 from the numpy array using a boolean array?

NumPy

import numpy as np
arr = np.array([10, 60, 30, 80, 50])
filtered = ???
print(filtered)

A arr[arr < 50]

B arr[arr == 50]

C arr[arr > 50]

D arr[arr >= 50]

💡 Hint

You want only elements strictly greater than 50.

arr > 50 creates a boolean array True only for elements greater than 50. Using it to index arr filters those elements.

🧠 Conceptual

expert

Understanding boolean array shape and dtype

Given a 2D numpy array, what is the shape and dtype of the boolean array created by comparing each element to zero?

NumPy

import numpy as np
arr = np.array([[1, -1], [0, 2]])
bool_arr = arr > 0
print(bool_arr.shape, bool_arr.dtype)

A (2, 2) bool

B (4,) int

C (2, 2) int

D (4,) bool

💡 Hint

The boolean array keeps the same shape as the original array.

The comparison creates a boolean array with the same shape as arr, and dtype is bool.