List Comprehension Question: One to Many Mapping?

Davo davbrow at gmail.com
Fri Aug 24 07:41:08 CEST 2007


On Aug 23, 9:24 pm, beginner <zyzhu2... at gmail.com> wrote:
> Hi All,
>
> How do I map a list to two lists with list comprehension?
>
> For example, if I have x=[ [1,2], [3,4] ]
>
> What I want is a new list of list that has four sub-lists:
>
> [[1,2], [f(1), f(2)], [3,4], [f(3), f(4)]]
>
> [1,2] is mapped to [1,2] and [f(1), f(2)] and [3,4] is mapped to
> [3,4], [f(3), f(4)].
>
> I just can't find any way to do that with list comprension. I ended up
> using a loop (untested code based on real code):
>
> l=[]
> for y in x:
>    l.append(y)
>    l.append([f(z) for z in y])
>
> Thanks,
> Geoffrey

This may be what you want:

l = [[y, [f(z) for z in y]] for y in x]

But It's a bit dense.  How about:
l=[]
for y in x:
    Fy = [f(z) for z in y]
    l.extend([y, Fy])

-- David




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