Iterate over all pairs of consecutive items in a list [duplicate]

Question

Given a list

l = [1, 7, 3, 5] 

I want to iterate over all pairs of consecutive list items (1,7), (7,3), (3,5), i.e.

for i in xrange(len(l) - 1):     x = l[i]     y = l[i + 1]     # do something 

I would like to do this in a more compact way, like

for x, y in someiterator(l): ... 

Is there a way to do do this using builtin Python iterators? I'm sure the itertools module should have a solution, but I just can't figure it out.

Answer 1

Just use zip

>>> l = [1, 7, 3, 5] >>> for first, second in zip(l, l[1:]): ...     print first, second ... 1 7 7 3 3 5 

If you use Python 2 (not suggested) you might consider using the izip function in itertools for very long lists where you don't want to create a new list.

import itertools  for first, second in itertools.izip(l, l[1:]):     ... 

Answer 2

Look at pairwise at itertools recipes: http://docs.python.org/2/library/itertools.html#recipes

Quoting from there:

def pairwise(iterable):     "s -> (s0,s1), (s1,s2), (s2, s3), ..."     a, b = tee(iterable)     next(b, None)     return izip(a, b) 

---

A General Version

A general version, that yields tuples of any given positive natural size, may look like that:

def nwise(iterable, n=2):                                                           iters = tee(iterable, n)                                                          for i, it in enumerate(iters):                                                        next(islice(it, i, i), None)                                                    return izip(*iters)