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How to turn a boolean array into index array in numpy?

Learn, how to turn a boolean array into index array in numpy in Python? By Pranit Sharma Last updated : October 10, 2023

NumPy is an abbreviated form of Numerical Python. It is used for different types of scientific operations in python. Numpy is a vast library in python which is used for almost every kind of scientific or mathematical operation. It is itself an array which is a collection of various methods and functions for processing the arrays.

Problem statement

Suppose that we are given a NumPy array and we need to focus on finding an efficient way to retrieve the integer indexes of locations in an array based on a condition that is true as opposed to the Boolean mask array.

Turning a boolean array into index array in numpy

We will use np.where() for this purpose and pass our specific condition to create a mask of values.

Let us understand with the help of an example,

Python program to turn a boolean array into index array in numpy

# Import numpy
import numpy as np

# Creating a numpy array
arr = np.arange(100,1,-1)

# Display original array
print("Original array:\n",arr,"\n")

# Creating a mask
res = np.where(arr&(arr-1) == 0)

# Display result
print("Result:\n",res)

Output

The output of the above program is:

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