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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Help with map python 2
Date: Sun, 04 Jan 2015 16:13:31 -0500
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Xref: csiph.com comp.lang.python:83220

On 1/4/2015 6:34 AM, flebber wrote:
> In repsonse to this question: Write a program that prints the first 100 members of the sequence 2, -3, 4, -5, 6, -7, 8.
>
> This is my solution it works but ugly.
>
> series = range(2,100)
> # answer = [(x,(y* -1)) for x, y in series[::2]]
> # print(answer)
> answer = []
> for item in series:
>      if item % 2 != 0:
>          answer.append(item * -1)
>      else:
>          answer.append(item)

for item in series:
     if item % 2:  # since result is a number, same as != 0
         item *= -1
     answer.append(item)

> print(answer)
>
> I know I should be better off doing this with map but cannot get it to work. I understand also that map returns a generator so this solution should only working in python2(correct me please if I am wrong).
>
> In [6]: map?
> Type:       builtin_function_or_method
> String Form:<built-in function map>
> Namespace:  Python builtin
> Docstring:
> map(function, sequence[, sequence, ...]) -> list
>
> Just getting something wrong
> list(map((lambda x: x * -1 if (x%2 != 0)), series))

In py2, map produces a list already.  In any case, above is syntax error 
without else clause.

map(lambda x: x * -1 if x%2 else x, series)

If you do not have a function already, a list comp is better.

[(-1*k if k%2 else k) for k in range(2, N)]

Change [] to () and you have a generator expression.

-- 
Terry Jan Reedy