Boolean Indexing

Summary: in this tutorial, you’ll learn how to access elements of a numpy array using boolean indexing.

Introduction to numpy array boolean indexing #

Numpy allows you to use an array of boolean values as an index of another array. Each element of the boolean array indicates whether or not to select the elements from the array.

If the value is True, the element of that index is selected. In case the value is False, the element of that index is not selected.

The following example uses boolean indexing to select elements of a numpy array using an array of boolean values:

import numpy as np

a = np.array([1, 2, 3])
b = np.array([True, True, False])
c = a[b]
print(c)Copy

Output:

[1 2]Copy

How it works.

First, create a new numpy array that includes three numbers from 1 to 3:

a = np.array([1, 2, 3])Copy

Second, create another numpy array with three boolean values True, True, and False:

b = np.array([True, True, False])Copy

Third, use the boolean array b as the index of the array a and assign the selected elements to the variable c:

c = a[b]Copy

Because the first and second elements of the array b are True, the a[b] returns a new array with the first and second elements of the array a.

Typically, you’ll use boolean indexing to filter an array. For example:

import numpy as np

a = np.arange(1, 10)
b = a > 5
print(b)

c = a[b]
print(c)Copy

Output:

[False False False False False  True  True  True  True]
[6 7 8 9]Copy

How it works.

First, create an array that has 9 numbers from 1 to 9 using the arange() function:

a = np.arange(1, 10)Copy

Second, create a boolean array from the following expression:

b = a > 5Copy

This expression compares each element of the array a with 5 and returns True if it is greater than 5 or False otherwise. The variable b is an array of boolean values:

[False False False False False  True  True  True  True]Copy

Third, use the array b as the index of array a and assign the result to the variable c:

c = a[b]Copy

The array c contains only numbers from array a, which are greater than 5.

Summary #

• Use boolean indexing to filter an array.