CODEWITHSUNDEEP
pythonlistpython-3.x
lecteur
lecteur

Reputation: 23

Conditionally replace elements in list1 with elements in list2

I am trying to merge two lists but I am getting an out-of-range error:

Example:

List1 = [1,2,0,7,0]
List2 = [3,6]

For i in range(len(List1)):
    if List1[i] == 0:
        List1[i] = List2[i]
        print(List1)

What's wrong with this code?

Upvotes: 0

Views: 142

Answers (2)

Stephen Rauch
Stephen Rauch

Reputation: 49794

To replace every element of List1 that is zero, with an element of List2,

Code:

List1 = [1, 2, 0, 7, 0]
List2 = [3, 6]

l2 = iter(List2)
List1 = [l if l != 0 else next(l2) for l in List1]

print(List1)

Produces:

[1, 2, 3, 7, 6]

Update from comments:

If List2 is possibly not long enough, one way around this is to specify a default value for next():

List1 = [1, 2, 0, 7, 0, 0]
List2 = [3, 6]

l2 = iter(List2)
List1 = [l if l != 0 else next(l2, None) for l in List1]

print(List1)

Which produces:

[1, 2, 3, 7, 6, None]

Upvotes: 0

Steven Summers
Steven Summers

Reputation: 5384

You seem to want to replace every instance of 0 from List1 with a value from List2 matching the index. The issue however is that for List1, 0 is at index 2 (indexes start at 0) but the is no item at index 2 for List2, hence the reason you get an IndexError

# Example
>>> lst = [1, 2, 3]
>>> lst[2]
3
>>> lst[3]
# IndexError: list index out of range

That explains what is wrong, now what would you like to do instead and maybe we can help.

Upvotes: 1

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