Re: Modifying every alternate element of a sequence


jm.suresh@xxxxxxxxxxxxxxxxx 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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