Can you set list indexes as variables in the original list?

Question

I'm trying to print a list of integers, one item per line, after swapping the position of the largest item with the last item. After swapping the items, they still print in their original position.

large = values[values.index(max(values))]
last = values[-1]

large, last = last, large

for i in values:
    print i

I'm sorry if this has already been answered, I haven't been able to find it yet.

@Chris_Rands I had looked at that before I posted my question and I wasn't able to fix my problem based on that. — CoffeeCat, Jul 03 '17 at 15:19

omri_saadon · Accepted Answer · 2017-07-03T14:03:20.987

6

You can reach it by:

max_index = values.index(max(values))
values[max_index], values[-1]= values[-1], values[max_index]

When you typed large, last = last, large you need to understand that large and last are variables that separated from the list values. means that even if you change them, the list won't change.

In order to update the list, you need to update the values inside the list as typed above.

edited Jul 03 '17 at 14:03

answered Jul 03 '17 at 13:51

omri_saadon

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OP seems new to `python`, maybe a list comprehension is not the best idea. – Sreetam Das Jul 03 '17 at 13:52
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it's not a list comprehension, it's a tuple pack/unpack to swap data idiomatically. – Jean-François Fabre Jul 03 '17 at 13:52
@Jean-FrançoisFabre I learnt something new today then. – Sreetam Das Jul 03 '17 at 13:54
@Jean-FrançoisFabre , Thanks for the note, updated to compute it once. – omri_saadon Jul 03 '17 at 13:55
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@omri_saadon and that got you some more upvotes :) – Jean-François Fabre Jul 03 '17 at 13:56
That worked, thank you. Do you know why it didn't work the way I did it? As far as I know, they're almost the same and I just wanted it to be easier to read. – CoffeeCat Jul 03 '17 at 13:57
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@Jean-FrançoisFabre [actually, no tuple is neither built nor unpacked](https://stackoverflow.com/questions/14836228/is-there-a-standardized-method-to-swap-two-variables-in-python/36199776#36199776) – Łukasz Rogalski Jul 03 '17 at 13:58
@ŁukaszRogalski that's very interesting! implementation detail, but explains why it's so fast & idiomatic. – Jean-François Fabre Jul 03 '17 at 14:01
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@CoffeeCat , Yes, because large and last points to a different memory location than the values inside the list. they have the same values, but changing large or last would not affect the list. You could do what Farmer did in his answer (setting large and last withing the list). – omri_saadon Jul 03 '17 at 14:01
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@CoffeeCat copying in a variable doesn't change the reference of the original. For that you have to address the list directly, as done in this good answer. – Jean-François Fabre Jul 03 '17 at 14:01

score 3 · Answer 2 · answered Jul 03 '17 at 13:54

3

You need to reassign large and last to where you would like them to be located in the list. After you make the swap add this:

values[values.index(max(values))] = large
values[-1] = last

answered Jul 03 '17 at 13:54

Pierre Delecto

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Can you set list indexes as variables in the original list?

2 Answers2