Numpy – Check If an Element is NaN

In this tutorial, we will look at how to check if an element in a Numpy array is a NaN (not a number) value or not with the help of some examples.

How to test if an element is NaN or not in a Numpy array?

You can use the numpy.isnan() function to check (element-wise) if values in a Numpy array are NaN or not. The following is the syntax –

# test for nan - pass scaler value or numpy array
np.isnan(a)

If you apply the numpy.isnan() function to a scalar value, it returns a boolean value (True if the value is NaN otherwise False). If you apply it to an array, it returns a boolean array.

Examples

Let’s now look at some examples of using the above function to test for NaN.

Example 1 – Check if a value is NaN or not using numpy.isnan()

First, let’s pass scaler values to the numpy.isnan() function.

Let’s create two variables – one containing a NaN value and the other containing a non-Nan value respectively and then apply the numpy.isnan() function on each of these values.

import numpy as np

# create two variables
a = 21
b = np.nan

# check if nan
print(np.isnan(a))
print(np.isnan(b))

Output:

False
True

We get False as the output for the value 21 and True as the output for the NaN value.

Example 2 – Element-wise check for NaN in a Numpy array using numpy.isnan()

If you apply the numpy.isnan() function on an array, it will return a boolean array containing with True for values that are NaN and False for the non-Nan values.

Let’s create a 1-D array and apply the numpy.isnan() function to it.

# create a numpy array
ar = np.array([1, 2, np.nan, 4, 5, np.nan, np.nan])
# element-wise check for nan value in ar
np.isnan(ar)

Output:

array([False, False,  True, False, False,  True,  True])

We get a boolean array as an output. You can see that in the boolean array we get True for NaN values in the original array and False for the other values.

Summary

In this tutorial, we looked at how we can use the numpy.isnan() function to check if an element is NaN or not in a Numpy array. Keep in mind that if you pass a scaler value, it returns a boolean value and if you pass a 1-D array it returns a boolean array.

You might also be interested in –

• Numpy – Check If Element is an Infinity or Not
• Check If Two Numpy Arrays are Equal
• Numpy – Set All Zeros to NaN