# List Comprehension Question: One to Many Mapping?

beginner zyzhu2000 at gmail.com
Fri Aug 24 07:44:50 CEST 2007

```On Aug 24, 12:41 am, Davo <davb... at gmail.com> wrote:
> 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- Hide quoted text -
>
> - Show quoted text -

This is exactly what I was looking for. Thanks.

```