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Home Interviews How to filter an array using NUmpy? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 23 Nov 2025
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To filter (select) values from a NumPy array, you use boolean indexing — create a boolean condition and apply it to the array.

Basic Example

import numpy as np

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

filtered = arr[arr > 3]
print(filtered)

Output

[4 5 6]

Filter Using Multiple Conditions

Use & (AND), | (OR), and ~ (NOT).
Parentheses are required.

arr = np.array([10, 20, 30, 40, 50])

filtered = arr[(arr >= 20) & (arr <= 40)]
print(filtered)

Output:

[20 30 40]

Filter with Another Array of Booleans

arr = np.array([5, 10, 15, 20])
mask = np.array([True, False, True, False])

filtered = arr[mask]
print(filtered)

Output:

[ 5 15 ]

Filter Strings

names = np.array(["Bob", "Alice", "John", "Anna"])
filtered = names[names == "Anna"]
print(filtered)

To filter strings containing a substring:

filtered = names[np.char.find(names, "n") >= 0]   # find returns -1 if not found
print(filtered)

Filter 2D Arrays

arr = np.array([[1, 2, 3],
                [4, 5, 6],
                [7, 8, 9]])

filtered = arr[arr > 5]
print(filtered)

Output:

[6 7 8 9]

Replace filtered values instead of selecting

arr = np.array([10, 20, 30, 40])
arr[arr > 25] = 0
print(arr)

Output:

[10 20  0  0]

Get indexes of matches

arr = np.array([3, 7, 2, 9, 7])
idx = np.where(arr == 7)
print(idx)

Result:

(array([1, 4], dtype=int64),)

Summary

Goal Code
Select values arr[arr > 3]
Select with conditions arr[(arr >= 10) & (arr <= 20)]
Boolean mask arr[mask]
Replace selected arr[arr > 10] = 0
Get indexes np.where(arr == x)
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