Lists aggregation

[mattia]

Il Tue, 17 Mar 2009 08:18:08 +0100, Peter Otten ha scritto:

> Mensanator wrote:
> 
>> On Mar 16, 1:40 pm, Peter Otten <__pete... at web.de> wrote:
>>> mattia wrote:
>>> > I have 2 lists, like:
>>> > l1 = [1,2,3]
>>> > l2 = [4,5]
>>> > now I want to obtain a this new list: l =
>>> > [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)] Then I'll have to transform
>>> > the values found in the new list. Now, some ideas (apart from the
>>> > double loop to aggregate each element of l1 with each element of
>>> > l2):
>>> > - I wanted to use the zip function, but the new list will not
>>> > aggregate (3,4) and (3,5)
>>> > - Once I've the new list, I'll apply a map function (e.g. the exp of
>>> > the values) to speed up the process
>>> > Some help?
>>>
>>> Why would you keep the intermediate list?
>>>
>>> With a list comprehension:
>>>
>>> >>> a = [1,2,3]
>>> >>> b = [4,5]
>>> >>> [x**y for x in a for y in b]
>>>
>>> [1, 1, 16, 32, 81, 243]
>>>
>>> With itertools:
>>>
>>> >>> from itertools import product, starmap from operator import pow
>>> >>> list(starmap(pow, product(a, b)))
>>>
>>> [1, 1, 16, 32, 81, 243]
>> 
>> That looks nothing like [(1,4),(1,5),(2,4),(2,5),(3,4),(3,5)].
> 
> The point of my post was that you don't have to calculate that list of
> tuples explicitly.
> 
> If you read the original post again you'll find that Mattia wanted that
> list only as an intermediate step to something else. He gave "the exp of
> values" as an example. As math.exp() only takes one argument I took this
> to mean "exponentiation", or **/pow() in Python.
> 
> Peter

Correct, and thanks for all the help.