Table of Contents
NumPy arrays can be filtered by using Boolean indexing to select only the elements that pass a certain condition. For example, you can use comparison operators to filter an array to only include values that are greater than, less than, or equal to a certain value. You can also use logical operators to filter multiple conditions at once. Finally, you can use the NumPy where() and MaskedArray functions to filter an array based on certain criteria.
You can use the following methods to filter the values in a NumPy array:
Method 1: Filter Values Based on One Condition
#filter for values less than 5 my_array[my_array < 5]
Method 2: Filter Values Using “OR” Condition
#filter for values less than 5 or greater than 9 my_array[(my_array < 5) | (my_array > 9)]
Method 3: Filter Values Using “AND” Condition
#filter for values greater than 5 and less than 9 my_array[(my_array > 5) & (my_array < 9)]
Method 4: Filter Values Contained in List
#filter for values that are equal to 2, 3, 5, or 12 my_array[np.in1d(my_array, [2, 3, 5, 12])]
This tutorial explains how to use each method in practice with the following NumPy array:
import numpy as np
#create NumPy array
my_array = np.array([1, 2, 2, 3, 5, 6, 7, 10, 12, 14])
#view NumPy array
my_array
array([ 1, 2, 2, 3, 5, 6, 7, 10, 12, 14])
Example 1: Filter Values Based on One Condition
The following code shows how to filter values in the NumPy array based on just one condition:
#filter for values less than 5 my_array[(my_array < 5)] array([1, 2, 2, 3]) #filter for values greater than 5 my_array[(my_array > 5)] array([ 6, 7, 10, 12, 14]) #filter for values equal to 5 my_array[(my_array == 5)] array([5])
Example 2: Filter Values Using “OR” Condition
The following code shows how to filter values in the NumPy array using an “OR” condition:
#filter for values less than 5 or greater than 9 my_array[(my_array < 5) | (my_array > 9)] array([ 1, 2, 2, 3, 10, 12, 14])
Example 3: Filter Values Using “AND” Condition
The following code shows how to filter values in the NumPy array using an “AND” condition:
#filter for values greater than 5 and less than 9 my_array[(my_array > 5) & (my_array < 9)] array([6, 7])
This filter returns the values in the NumPy array that are greater than 5 and less than 9.
Example 4: Filter Values Contained in List
The following code shows how to filter values in the NumPy array that are contained in a list:
#filter for values that are equal to 2, 3, 5, or 12 my_array[np.in1d(my_array, [2, 3, 5, 12])] array([ 2, 2, 3, 5, 12])
This filter returns only the values that are equal to 2, 3, 5, or 12.
Note: You can find the complete documentation for the NumPy in1d() function .
The following tutorials explain how to perform other common filtering operations in Python: