[FIXED] Replace data in an array with numpy

Issue

I am using a digital elevation model as an array, and I want to create a new array with only the pixels below a certain value. I tried using a for loop.

Here is the operation with simpler data :

import numpy as np

array1 = array2 = np.arange(9).reshape(3,3)
array2 = array2.astype("float") #np.nan doesn't work with integers

for row in array2: 
    for item in row:
        if item > 3.0:
            item=np.nan

print(np.array_equal(array1,array2))

The arrays remain equal after the loop is over. Do you know why the values won't be replaced?

Solution

You only change item you need insert item in array2, you can use np.where like below:

>>> array1 = np.arange(9).reshape(3,3)
>>> array2 = np.where(array1 > 3, np.nan, array1)
>>> array2
array([[ 0.,  1.,  2.],
       [ 3., nan, nan],
       [nan, nan, nan]])

>>> np.array_equal(array1, array2)
False

Answered By - user1740577