Pairwise Iteration

The zip function in Python can be used in some really interesting ways.

One of those ways is for iterating through an iterable in consecutive pairs.

Consider this iterable:

l = [1, 2, 3, 4, 5, 6]

Suppose we need to iterate through this list pairwise as follows:

(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)

We could certain set up a loop of indices and do it the "normal" way, but a much more Pythonic way of doing this would be to use sequence slicing and the zip function as follows:

for t in zip(l, l[1:]):

    print(t)

which will give us the result:

(1, 2)(2, 3)(3, 4)(4, 5)(5, 6)

We can tweak this further to allow us to iterate thus:

(1, 2), (3, 4), (5, 6)

by writing it this way:

for t in zip(l[::2], l[1::2]):

    print(t)

which gives us the desired result:

(1, 2)(3, 4)(5, 6)

Of course we can expand on this to iterate in 3-tuple, 4-tuple, etc.

For example:

l = range(10)for t in zip(l, l[1::], l[2::]):

    print(t)

will result in this output:

(0, 1, 2)(1, 2, 3)(2, 3, 4)(3, 4, 5)(4, 5, 6)(5, 6, 7)(6, 7, 8)(7, 8, 9)

while this:

l = range(15)
for t in zip(l[::3], l[1::3], l[2::3]):
    print(t)

will result in this output:

(0, 1, 2)(3, 4, 5)(6, 7, 8)(9, 10, 11)(12, 13, 14)

For a three character function, zip packs a lot of punch!