Book Image

NumPy: Beginner's Guide

By : Ivan Idris
Book Image

NumPy: Beginner's Guide

By: Ivan Idris

Overview of this book

Table of Contents (21 chapters)
NumPy Beginner's Guide Third Edition
Credits
About the Author
About the Reviewers
www.PacktPub.com
Preface
NumPy Functions' References
Index

Time for action – extracting elements from an array


Let's extract the even elements of an array:

  1. Create the array with the arange() function:

    a = np.arange(7)
  2. Create the condition that selects the even elements:

    condition = (a % 2) == 0
  3. Extract the even elements using our condition with the extract() function:

    print("Even numbers", np.extract(condition, a))

    This gives us the even numbers as required (np.extract(condition, a) is equivalent to a[np.where(condition)[0]]):

    Even numbers [0 2 4 6]
    
  4. Select non-zero values with the nonzero() function:

    print("Non zero", np.nonzero(a))

    This prints all the non-zero values of the array:

    Non zero (array([1, 2, 3, 4, 5, 6]),)
    

What just happened?

We extracted the even elements from an array using a Boolean condition with the NumPy extract() function (see extracted.py):

from __future__ import print_function
import numpy as np

a = np.arange(7)
condition = (a % 2) == 0
print("Even numbers", np.extract(condition, a))
print("Non zero", np.nonzero(a))