Modify a part of numpy array based upon a condition

Question

I have a numpy array with zeros and non-zeros and shape (10,10). To a subpart of this array I need to add a certain value, where initial value is not zero.

a[2:7,2:7] += 0.5 #But with a condition that a[a!=0]

Currently, I do it in a rather cumbersome way, by first making a copy of the array and modifying the second array consistently and then copying back to the first.

b = a.copy()
b[b!=0] = 1
b[2:7,2:7] *= 0.5
b[b ==1] =0
a += b

Is there more elegant way to achieve this?

b = a[2:7,2:7] and then b[b!=0] += 0.5. Test it with print(a). — Thomas Kühn
– Thomas Kühn, Commented Feb 16, 2019 at 11:47
I forgot that unless you make a copy, b would still refer to a. Too much mutability, but sometime you can use for your good. — KarateKid
– KarateKid, Commented Feb 16, 2019 at 11:54

KarateKid · Accepted Answer · 2019-02-16 11:58:49Z

2

As Thomas Kühn, correctly wrote in the comment, its good enough to create a reference to that subpart of the array and modify it. So the following does the job.

b = a[2:7,2:7]
b[b!=0] += 0.5

answered Feb 16, 2019 at 11:58

KarateKid

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