numpy.where() in Python with Examples

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

Basic Syntax

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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]) 

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]) 

The output should be:

[[4 5 6]
[7 8 9]]
All indices of elements <7
(array([0, 0, 0]), array([0, 1, 2]))
All elements which are <7
[4 5 6]