Python: How to replace values in an array by values of another array if smaller than?

Question

I'm trying to compare two lists and replace the values of the first one by the values of the second one only if they are smaller.

I tried the smaller than function from python but this seems to replace the whole list.

A = [7 , 10 , 2, 5 , 9]
B = [8 , 12 , 1, 4 , 7]

A[A < B] = B

This replaces list A by list B, probably because it compares the whole list. I would want only the values replaced that are smaller than the ones in B resulting in:

[8 , 12 , 2 , 5 , 9]

Are you sure that A[A<B] = B doesn't just assign B to A[1]? — Mad Physicist
– Mad Physicist, Commented Oct 24, 2019 at 15:06
What does this have to do with numpy? If you're using numpy, please show a proper example. — Mad Physicist
– Mad Physicist, Commented Oct 24, 2019 at 15:06
Yes, it completely replaces A. About the numpy that was a mistake, sorry. — Michiel
– Michiel, Commented Oct 24, 2019 at 15:12
I've added a non numpy solution if you want to avoid the import — Mad Physicist
– Mad Physicist, Commented Oct 24, 2019 at 15:25

Dani Mesejo · Accepted Answer · 2019-10-24 15:06:02Z

4

Do:

import numpy as np

A = np.array([7, 10, 2, 5, 9])
B = np.array([8, 12, 1, 4, 7])

A[A < B] = B[A < B]
print(A)

Output

[ 8 12  2  5  9]

answered Oct 24, 2019 at 15:06

Dani Mesejo

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3 Comments

Michiel Over a year ago

This looks like the right answer. However, when I try this the firs number of A stays a 7.

Dani Mesejo Over a year ago

@Michiel I cannot reproduce, I ran this on my computer and gives the same output as in the answer. What numpy version are you using?

Michiel Over a year ago

I am using 1.13.3. Found my mistake, I forgot to add np.array. Thanks for the help!

RomanPerekhrest · Accepted Answer · 2019-10-24 15:12:32Z

2

With numpy.place - to change elements of an array in-place based on conditional:

import numpy as np

a = np.array([7 , 10 , 2, 5 , 9])
b = np.array([8 , 12 , 1, 4 , 7])
np.place(a, a < b, b)
print(a)    # [ 8 12  2  5  9]

answered Oct 24, 2019 at 15:12

RomanPerekhrest

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Comments

Mad Physicist · Accepted Answer · 2019-10-24 15:24:41Z

1

If you want to keep this in vanilla python, you can do something like:

A[:] = [max(a, b) for a, b in zip(A, B)]

This does a true in-place operation, such that id(A) does not change. A simpler approach might be to just reassign A:

A = [max(a, b) for a, b in zip(A, B)]

answered Oct 24, 2019 at 15:24

Mad Physicist

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