Replace multiple list elements at the same time (Python)

Question

Given two lists (a and b), I'd like to replace three elements of list 'a' with three elements of list 'b'. Currently I am using an expression like this:

a[0], a[5], a[7] = b[11], b[99], b[2]

As I need to do such operations very frequently with lots of different arrays I am wondering if there is a more compact solution for this problem (the number of elements I need to replace is always 3 though). I was thinking about something like:

a[0,5,7] = b[11,99,2]

Which obviously does not work.

Ashish Ranjan · Accepted Answer · 2017-10-28 20:50:13Z

2

If you've a python list you can do something like this :

toReplace = [0,5,7]
targetIndices = [11, 99, 2]

for i,j in zip(toReplace, targetIndices): a[i] = b[j]

If you've a numpy array, it's even simpler :

a[toReplace] = b[targetIndices]
#i.e, a[[0,5,7]] = b[[11, 99, 2]]

edited Oct 28, 2017 at 20:50

answered Oct 28, 2017 at 20:32

Ashish Ranjan

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2 Comments

Istvanb Over a year ago

This simplifies the code when you need to replace fair amount of elements, but not when you need to replace only a couple of them. I have specified my question more, I always need to replace three element at a given time.

Ashish Ranjan Over a year ago

@Istvanb if you've a python list, then that's all you can do, just in case you've a numpy array you can do something like you want to do, see the updated answer.

Erlinska · Accepted Answer · 2017-10-28 20:36:12Z

1

There might be some better solutions but this does the trick:

ind1 = [0,5,7] 
ind2 = [11,99,2]

for i in range(len(ind1)):
    a[ind1[i]]=b[ind2[i]]

answered Oct 28, 2017 at 20:36

Erlinska

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1 Comment

Istvanb Over a year ago

This simplifies the code when you need to replace fair amount of elements, but not when you need to replace only a couple of them. I have specified my question more, I always need to replace three element at a given time.

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