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: > [EMAIL PROTECTED] 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
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