NumPyFilter Array

Filtering Arrays

Getting some elements out of an existing array and creating a new array out of them is calledfiltering.

In NumPy, you filter an array using aboolean index list.

Aboolean index list is a list of booleans corresponding to indexes in the array.

If the value at an index isTrue that element is contained in the filtered array, if the value at that index isFalse that element is excluded from the filtered array.

Example

Create an array from the elements on index 0 and 2:

import numpy as np

arr = np.array([41, 42, 43, 44])

x = [True, False, True, False]

newarr = arr[x]

print(newarr)

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The example above will return[41, 43], why?

Because the new array contains only the values where the filter array had the valueTrue, in this case, index 0 and 2.

Creating the Filter Array

In the example above we hard-coded theTrue andFalse values, but the common use is to create a filter array based on conditions.

Example

Create a filter array that will return only values higher than 42:

import numpy as np

arr = np.array([41, 42, 43, 44])

# Create an empty list
filter_arr = []

# go through each element in arr
for element in arr:
# if the element is higher than 42, set the value to True, otherwise False:
if element > 42:
filter_arr.append(True)
else:
filter_arr.append(False)

newarr = arr[filter_arr]

print(filter_arr)
print(newarr)

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Example

Create a filter array that will return only even elements from the original array:

import numpy as np

arr = np.array([1, 2, 3, 4, 5, 6, 7])

# Create an empty list
filter_arr = []

# go through each element in arr
for element in arr:
# if the element is completely divisble by 2, set the value to True, otherwise False
if element % 2 == 0:
filter_arr.append(True)
else:
filter_arr.append(False)

newarr = arr[filter_arr]

print(filter_arr)
print(newarr)

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