numpy.where() in Python with Examples

February 9, 2019

numpy.where() function in Python returns the indices of items in the input array when the given condition is satisfied.

Basic Syntax

Following is the basic syntax for np.where() function:

numpy.where(condition[, x, y])

And the parameters are:

Parameter	Description
condition	When condition is true it yields x otherwise y. Where `x,y` are the values from which to choose x and y

Return Value

When both x and y is provided then if the condition becomes true it will return elements of x otherwise elements of y.

Example

Following are the examples for numpy.where() function.

Example 1:

# Python program illustrating where() function  
  
import numpy as np 
  
# items array. 
items = np.array([[0, 10, 20], [30, 40, 50]]) 

# print items array
print(items) 
  
# now print item elements which are greater than 20
print ('All indices of elements >20') 
  
result = np.where(items>20) 
print(result) 
  
print("All elements which are >20") 
print(items[result])

# Python program illustrating where() function

import numpy as np

# items array.

items = np.array([[0, 10, 20], [30, 40, 50]])

# print items array

print(items)

# now print item elements which are greater than 20

print ('All indices of elements >20')

result = np.where(items>20)

print(result)

print("All elements which are >20")

print(items[result])

The output should be:

[[ 0 10 20]
[30 40 50]]
All indices of elements >20
(array([1, 1, 1]), array([0, 1, 2]))
All elements which are >20
[30 40 50]

Example 2:

# Python program illustrating where() function  
  
import numpy as np 
  
# items array. 
items = np.arange(4,10).reshape(2,3) 

# print items array
print(items) 
  
# now print item elements which are greater than 20
print ('All indices of elements <7') 
  
result = np.where(items<7) 
print(result) 
  
print("All elements which are <7") 
print(items[result])