[Tutor] Iterating over two sequences in "parallel"

Dave Angel davea at ieee.org
Sat Nov 28 13:59:04 CET 2009

Jose Amoreira wrote:
> Hi!
> I want to process corresponding elements of two lists, sequentially. Call the 
> lists list1 and list2, and assume they have equal lengths. I can do something 
> like
> for index in range(len(list1)):
>     process(list1[index], list2[index])
> But I find it somehow rather ugly, because we generate yet another an list for 
> the index, when we already have the two we want to process. I know we can use 
> xrange, but still I find it awkward...
> Instead of the above snippet, I am considering something like
> while list1:
>     process(list1.pop(), list2.pop())
> But this has the side effect of emptying both lists, which may not be 
> convenient. Of course we can make backup copies of the lists if needed, but we 
> are then recovering the previous method ugliness...
> Do you guys have any suggestions regarding this? Thanks
> Jose Amoreira
(following is untested, and from memory, so I may not have it perfect.  
But it's close)

This is exactly what zip() is for.
       zip(list1, list2)
yields a new list that consists of tuples taken in sequence from those 
two lists.  So
      for item1, item2 in zip(list1, list2):
             process(item1, item2)

And if the lists are large, use  itertools.izip() which works the same, 
but produces an iterator.

Note that if the lists are not the same length, I think it stops when 
the shorter one ends.


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