NumPy - where() function

The NumPy where() function returns elements chosen from x or y depending on condition. When only condition is provided, the function returns the indices of elements of the given array which satisfies the condition.

Syntax

numpy.where(condition, x, y)

Parameters

`condition`	`Required.` Specify array_like, bool. Where True, yield x, otherwise yield y.
`x, y`	`Optional.` Specify array_like values from which to choose. x, y and condition need to be broadcastable to some shape.

Return Value

Returns an array with elements from x where condition is True, and elements from y elsewhere.

Example:

In the example below, where() function is used to replace all negative elements with 0 from an array.

import numpy as np

x = np.arange(-2, 5)

#replacing all negative elements with 0
y = np.where(x > 0, x, 0)

#displaying the content of x and y
print("x contains:", x)
print("y contains:", y)

The output of the above code will be:

x contains: [-2 -1  0  1  2  3  4]
y contains: [0 0 0 1 2 3 4]

Example:

In this example, where() function is used to choose elements from two array based on a given condition.

import numpy as np

x = np.asarray([[10, 20], [30, 40]])
y = np.asarray([[15, 15], [25, 25]])

#applying where condition
z = np.where(x > y, x, y)

#displaying the content of x, y and z
print("x =")
print(x)
print("\ny =")
print(y)
print("\nz =")
print(z)

The output of the above code will be:

x =
[[10 20]
 [30 40]]

y =
[[15 15]
 [25 25]]

z =
[[15 20]
 [30 40]]

Example:

When only condition is provided, the function returns the indices of elements of the given array which satisfies the condition. Consider the following example:

import numpy as np

x = np.asarray([10, 20, 30, 40, 50, 60])

#applying where condition
y = np.where(x > 35)

#displaying the result
print("x =", x)
print("y =", y)
print("x[y] =", x[y])

The output of the above code will be:

x = [10 20 30 40 50 60]
y = (array([3, 4, 5]),)
x[y] = [40 50 60]

Example:

Condition can be passed as array as well. Consider the following example.

import numpy as np

x = np.asarray([[10, 20], [30, 40]])
y = np.asarray([[15, 15], [25, 25]])
cond = np.asarray([[True, True], [False, False]])

#applying where condition
z = np.where(cond, x, y)

#displaying the content of x, y and z
print("x =")
print(x)
print("\ny =")
print(y)
print("\nz =")
print(z)

The output of the above code will be:

x =
[[10 20]
 [30 40]]

y =
[[15 15]
 [25 25]]

z =
[[10 20]
 [25 25]]

❮ NumPy - Functions