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numpy.where() in Python
  • Difficulty Level : Medium
  • Last Updated : 03 Dec, 2020

The numpy.where() function returns the indices of elements in an input array where the given condition is satisfied.

Syntax :numpy.where(condition[, x, y])
condition : When True, yield x, otherwise yield y.
x, y : Values from which to choose. x, y and condition need to be broadcastable to some shape.

out : [ndarray or tuple of ndarrays] If both x and y are specified, the output array contains elements of x where condition is True, and elements from y elsewhere.

If only condition is given, return the tuple condition.nonzero(), the indices where condition is True.

Code #1:

# Python program explaining 
# where() function 
import numpy as np
np.where([[True, False], [True, True]],
         [[1, 2], [3, 4]], [[5, 6], [7, 8]])

Output :

array([[1, 6],
       [3, 4]])


Code #2:

# Python program explaining 
# where() function 
import numpy as np
# a is an array of integers.
a = np.array([[1, 2, 3], [4, 5, 6]])
print ('Indices of elements <4')
b = np.where(a<4)
print("Elements which are <4")

Output :

[[1 2 3]
 [4 5 6]]

Indices of elements <4
(array([0, 0, 0], dtype=int64), array([0, 1, 2], dtype=int64))

Elements which are <4
array([1, 2, 3])

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