# Help with map python 2

Terry Reedy tjreedy at udel.edu
Sun Jan 4 22:13:31 CET 2015

```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]]
> for item in series:
>      if item % 2 != 0:
>      else:

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

>
> 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

```