Question

I can loop through a Python (v.2.6) list without specifying indices, using the following “Pythonic” syntax:

the_list = [2, 3, 5, 7, 11, 13]
for item in the_list:
    print item + 2

But if I want to perform an operation on two consecutive indices, I think I then have to specify index numbers, with a corresponding change to the range of the for loop:

the_list = [2, 3, 5, 7, 11, 13]
for i in xrange(len(the_list)-1):
    print the_list[i] + the_list[i+1]

Is that correct? Or is there a way to remain Pythonic and avoid using the expression xrange(len(the_list)-1)?

Answer 1

I’m fond of the pairwise recipe as listed in the docs to the itertools module:

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

Then…

for x, x_next in pairwise(the_list):
    ...

You can also just zip the list with a slice of itself:

for x, x_next in zip(the_list, the_list[1:]):
    ...