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... -- https://mail.python.org/mailman/listinfo/python-list
