NumPy where()

where() Syntax

The syntax of where() is:

numpy.where(condition, x, y)

where() Arguments

The where() method takes three arguments:

• condition - a boolean or an array
• x - value to take if the condition is True
• y - value to take if the condition is False

We can also pass a single argument to np.where(). To learn about it, visit np.where() with a single argument section below.

where() Return Value

The where() method returns a new NumPy array.

Example 1: numpy.where() With Two Arrays

Output

[1 2 30 40]

Example 2: numpy.where() with Operation

We can also use numpy.where() to perform operations on array elements.

Output

[1 2 3 4]

Example 3: where() with Array Condition

We can use array_like objects (such as lists, arrays etc.) as a condition in the where() method.

Output

[[1 2]
 [-3 -4]]

[[1 -2]
 [-3 4]]

Example 4: where() with Multiple Conditions

The test condition in a where() method may have multiple conditions.

We use

• the | operator to perform OR operation on multiple conditions
• the & operator to perform AND operation on multiple conditions

Output

[1 0 0 0 0 0 7]
[0 0 3 4 5 0 0]

Example 5: where() with Only One Argument

If we pass a single argument (test condition) to numpy.where(), it tells us where in a given array the given condition is met by returning the indices.

Output

(array([4, 5, 6, 7], dtype=int64), )