Modifying every alternate element of a sequence

[jm.suresh@no.spam.gmail.com]

Wow, I was in fact searching for this syntax in the python tutorial. It
is missing there.
 Is there a reference page which documents all possible list
comprehensions.
--
Suresh
Leo Kislov wrote:
> jm.suresh at no.spam.gmail.com wrote:
> > I have a list of numbers and I want to build another list with every
> > second element multiplied by -1.
> >
> > input = [1,2,3,4,5,6]
> > wanted = [1,-2,3,-4,5,-6]
> >
> > I can implement it like this:
> >
> > input = range(3,12)
> > wanted = []
> > for (i,v) in enumerate(input):
> >     if i%2 == 0:
> >         wanted.append(v)
> >     else:
> >         wanted.append(-v)
> >
> > But is there any other better way to do this.
>
> Use slices:
>
> input[1::2] = [-item for item in input[1::2]]
>
> If you don't want to do it in-place, just make a copy:
>
> wanted = input[:]
> wanted[1::2] = [-item for item in wanted[1::2]]
> 
>   -- Leo