Replace all elements of NumPy array that are greater than some v

Replace all elements of NumPy array that are greater than some value

I have a 2D NumPy array. How do I replace all values in it greater than a threshold T = 255 with a value x = 255? A slow for-loop based method would be:

# arr = arr.copy()  # Optionally, do not modify original arr.

for i in range(arr.shape[0]):
    for j in range(arr.shape[1]):
        if arr[i, j] > 255:
            arr[i, j] = x

Answers

You can avoid using a slow for loop by utilizing NumPy's vectorized operations. Here's a faster and more efficient way to replace all values in the 2D array that are greater than a threshold T = 255 with x = 255:

import numpy as np

# Example array
arr = np.array([[100, 256, 200], [300, 150, 400]])

# Replace values greater than 255 with 255
arr[arr > 255] = 255

print(arr)

Explanation:

• arr > 255 creates a boolean mask where values greater than 255 are True.
• arr[arr > 255] = 255 replaces all the values in arr that are True in the mask with 255.

This method is efficient because it leverages NumPy's ability to perform element-wise operations in bulk without the need for explicit loops.