Is there a way to have parallel for-each loops?

Question

Let's say I have 2 lists in Python and I want to loop through each one in parallel - e.g. do something with element 1 for both lists, do something with element 2 for both lists... I know that I can do this by using an index:

for listIndex in range(len(list1)):
   doSomething(list1[listIndex])
   doSomething(list2[listIndex])

But is there a way to do this more intuitively, with a foreach loop? Something like for list1Value in list1, list2Value in list2...?

I've currently run into this situation in Python, but this is a longstanding question and I'd be interested to know if you can do this in any language. (I just assumed that Python is the most likely to have a method of dealing with this.)

Answer 1

Something like this?

for (a,b) in zip(list1, list2):
  doSomething(a)
  doSomething(b)

Though if doSomething() isn't doing I/O or updating global state, and it just works on one of the elements at a time, the order doesn't matter so you could just use chain() (from itertools):

for x in chain(list1, list2):
  doSomething(x)

Apropos, from itertools import * is something I do very often. Consider izip() instead of using the zip() I gave above. Also look into izip_longest(), izip(count(), lst), etc. Welcome to functional programming. :-)

Oh, and zipping also works with more "columns":

for idx, a, b, c in izip(count(), A, B, C):
  ...

Answer 2

That will depend on the language. Python actually has a rather simple method for that:

a = (0,1,2,3,4,5,6,7,8,9)
b = "ABCDEFGHIJ"
for pair in zip(a,b):
  print("%d => %s" % pair)

Answer 3

Use zip or itertools.izip for this:

for item1, item2 in zip(iterable1, iterable2):
    # process the items in parallel

itertools.izip in Python < 3 and zip in Python ≥ 3 return iterators; i.e. they provide tuples of pairs (or triplets, quartets etc) on request. Python < 3 zip creates a list of tuples, so the memory requirements could be large if the smallest of the sequences is quite long.