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

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.

Solution 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:]):
    ...

Solution 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)