zip list, variables
Peter Otten
__peter__ at web.de
Wed Nov 20 05:38:36 EST 2013
flebber wrote:
> If
>
> c = map(sum, zip([1, 2, 3], [4, 5, 6]))
>
> c
> Out[7]: [5, 7, 9]
>
> why then can't I do this?
>
> a = ([1, 2], [3, 4])
>
> b = ([5, 6], [7, 8])
>
> c = map(sum, zip(a, b))
>
---------------------------------------------------------------------------
> TypeError Traceback (most recent call
> last) <ipython-input-3-cc046c85514b> in <module>()
> ----> 1 c = map(sum, zip(a, b))
>
> TypeError: unsupported operand type(s) for +: 'int' and 'list'
>
> How can I do this legally?
You are obscuring the issue with your map-zippery. The initial value of
sum() is 0, so if you want to "sum" lists you have to provide a start value,
typically an empty list:
>>> sum([[1],[2]])
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'list'
>>> sum([[1],[2]], [])
[1, 2]
Applying that to your example:
>>> def list_sum(items):
... return sum(items, [])
...
>>> map(list_sum, zip(a, b))
[[1, 2, 5, 6], [3, 4, 7, 8]]
Alternatively, reduce() does not require an initial value:
>>> map(functools.partial(reduce, operator.add), zip(a, b))
[[1, 2, 5, 6], [3, 4, 7, 8]]
But doing it with a list comprehension is the most pythonic solution here...
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