# Help with map python 2

Terry Reedy tjreedy at udel.edu
Tue Jan 6 00:28:40 CET 2015

On 1/5/2015 6:33 AM, flebber wrote:
>
>> You could do what mathematicians do when they deal with alternating
>> signs: they raise -1 to the power of the index to get an appropriate
>> multiplier.
>>
>>     >>> [ n * (-1) ** n for n in range(10) ]
>>     [0, -1, 2, -3, 4, -5, 6, -7, 8, -9]

Mathematicians operating in the timeless Platonic universe of
mathemtical forms are not concerned with such droll issues as
computation time.  I think most regard '(-1)**n' as an abbreviation for
'(-1 if odd(n) else 1)', where odd(n) is regarded as an O(1) operation,
not as an indication to actually raise -1 to an arbitrarily large power.
which could take an arbitrarily long time.

>> Or you could do here what you attempt to do with map below. See below.
>>
>
>>
>> You are trying to use a binary expression. There are no binary
>> expressions. Add an else branch to make it ternary:
>>
>>     lambda x : x if x % 2 == 0 else -x
>>
>> But never mind the number of branches, the serious point is that you
>> didn't specify a value for when the condition is not true. It doesn't
>> make sense without that.
>>
>> There's nothing wrong with a list comprehension, or the corresponding
>> generator expression if you want a generator. It's fine. Map's fine.
>
> Thanks Jussi  I really like the multiplier solution.
>

--
Terry Jan Reedy