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)Code language: Python (python)

Output:

[1 2]Code language: Python (python)

How it works.

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

a = np.array([1, 2, 3])Code language: Python (python)

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

b = np.array([True, True, False])Code language: Python (python)

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]Code language: Python (python)

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)Code language: Python (python)

Output:

[False False False False False  True  True  True  True]
[6 7 8 9]Code language: Python (python)

How it works.

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

a = np.arange(1, 10)Code language: Python (python)

Second, create a boolean array from the following expression:

b = a > 5Code language: Python (python)

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]Code language: Python (python)

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

c = a[b]Code language: Python (python)

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

Summary

• Use boolean indexing to filter an array.