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):
  ...