# List Comprehension Question: One to Many Mapping?

beginner zyzhu2000 at gmail.com
Fri Aug 24 15:26:22 CEST 2007

```On Aug 24, 12:44 am, beginner <zyzhu2... at gmail.com> wrote:
> On Aug 24, 12:41 am, Davo <davb... at gmail.com> wrote:
>
>
>
>
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> > 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.- Hide quoted text -
>
> - Show quoted text -

On second thought, that will generate one additional level. So it is
not what I am looking for.

```