CODEWITHSUNDEEP
pythonarraysnumpy
pbreach
pbreach

Reputation: 16997

How to "remove" mask from numpy array after performing operations?

I have a 2D numpy array that I need to mask based on a condition so that I can apply an operation to the masked array then revert the masked values back to the original.

For example:

import numpy as np

array = np.random.random((3,3))
condition = np.random.randint(0, 2, (3,3))
masked = np.ma.array(array, mask=condition)

masked += 2.0

But how can I change the masked values back to the original and "remove" the mask after applying a given operation to the masked array?

The reason why I need to do this is that I am generating a boolean array based on a set of conditions and I need to modify the elements of the array that satisfy the condition.

I could use boolean indexing to do this with a 1D array, but with the 2D array I need to retain its original shape ie. not return a 1D array with only the values satisfying the condition(s).

Upvotes: 5

Views: 16771

Answers (2)

G M
G M

Reputation: 22459

The accepted answer doesn't answer the question. Assigning the mask to False works in some cases but with many algorithms that do not support masked arrays (e.g. scipy.linalg.lstsq()) you will experience an error like this:

ValueError: masked arrays are not supported

You can test if you array has still the attribute mask like this:

import numpy as np

array = np.random.random((3,3))
condition = np.random.randint(0, 2, (3,3))
masked = np.ma.array(array, mask=condition)

masked += 2.0

masked.mask = False
hasattr(masked, 'mask')
>> True

The only way to really get rid of the mask is by assigning a variable only to the data of the masked array:

masked = masked.data 
hasattr(masked, 'mask')
>> False

Upvotes: 10

John Zwinck
John Zwinck

Reputation: 249153

You already have it: it's called array!

This is because while masked makes sure you only increment certain values in the matrix, the data is never actually copied. So once your code executes, array has the elements at condition incremented, and the rest remain unchanged.

Upvotes: 8

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