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I am fairly new to Python and I'm having trouble figuring out how to apply a list comprehension to part of a nested list (specifically at the index level).

For instance, if I have the following:

my_list = [[1,2], [3,7], [6,9], [4,3]]

new_list = [[i*2 for i in sublist] for sublist in my_list]

How would I alter my list comprehension to only apply the operation to index 1 of each sublist? I have experimented quite a bit with no success.

donopj2
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    There's no condition in your code. Please state what kind of output you'd like to get for this list (which contains syntax errors, btw.). – Fred Foo Dec 06 '11 at 11:49

3 Answers3

2

A more general version of mac's:

>>> my_list = [[1,2], [3,7], [6,9], [4,3]]
>>> new_list = [[v*2 if i==0 else v for i,v in enumerate(sublist)] for sublist in my_list]
>>> new_list
[[2, 2], [6, 7], [12, 9], [8, 3]]
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Dan D.
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1

Are you looking for this?

>>> my_list = [[1,2], [3,7], [6,9], [4,3]]
>>> [[sublist[0] * 2, sublist[1]] for sublist in my_list]
[[2, 2], [6, 7], [12, 9], [8, 3]]

EDIT: The above solution wouldn't scale well if you had sublists of many elements. If this is the case for you, an alternative could be use mapping:

>>> my_list = [[1,2], [3,7], [6,9], [4,3]]
>>> def double_first(list_):
...     list_[0] *= 2
...     return list_
... 
>>> map(double_first, my_list)
[[2, 2], [6, 7], [12, 9], [8, 3]]

EDIT2: The solution in my first edit allows to implement any type of manipulation on the sublists, but if the operation is basic and only dependent from the index of the sublist, Dan's solution will perform faster.

HTH!

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mac
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0

you mean something like this:

my_list = [[1,2], [3,7], [6,9], [4,3]]

new_list = [sublist[0]*2 for sublist in my_list]

Output: 
    new_list == [2, 6, 12, 8]

also, you forgot to put commas between your sublists (Fixed in my answer)

I am assuming that by 'index 1' you mean the first element.
If you actually mean the second item (which is index 1) you will use sublist[1] instead of sublist[0].

Serdalis
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  • Thanks. I realized my syntax errors shortly after posting (and corrected) - just wanted to type up simple code so i could understand the concept. I can't believe I didn't see it before. Much appreciated – donopj2 Dec 06 '11 at 11:55
  • @donopj22 no problem, did this do what you wanted? or did you want something different? – Serdalis Dec 06 '11 at 11:56
  • Yes, all the answers were actually very helpful. I Appreciate everyone's input. – donopj2 Dec 06 '11 at 12:36