Get Pairs of List Python

Question

c = [1,2,3,4,5,6,7,8,9,10]

for a,b in func(c):
    doSomething()

So func() have to return (1,2) (2,3) (3,4) ... (8,9) (9,10)

Is there a elegant method in python 2.7 to achieve this?

Answer 1

Sure, there are many ways. Simplest:

def func(alist):
    return zip(alist, alist[1:])

This spends a lot of memory in Python 2, since zip makes an actual list and so does the slicing. There are several alternatives focused on generators that offer memory savings, such as a very simple:

def func(alist):
    it = iter(alist)
    old = next(it, None)
    for new in it:
        yield old, new
        old = new

Or you can get fancier deploying the powerful itertools instead, as in the pairwise recipe proposed by @HughBothwell .

Answer 2

The itertools documentation has a recipe for this:

from itertools import tee, izip

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

then

for a,b in pairwise(c):
    doSomething(a, b)

Answer 3

There are many ways

>>> a = [1,2,3,4,5,6,7,8,9,10]
>>> list(zip(a,a[1:]))
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 8), (8, 9), (9, 10)]

The other ways are

• [(a[i],a[i+1]) for i in range(len(a)-1)]

As you wanter a function you can do

func = lambda a : list(zip(a,a[1:]))

Answer 4

You can use zip:

>>> def pair(sample_list):
...    return zip(sample_list,sample_list[1:])
... 
>>> pair(a)
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 8), (8, 9), (9, 10)]

Or with iter() that return an iterator , so you can use next() attribute of iterator in a list comprehension to get the proper pairs , note that in both recipes the second object need to be slice from first element to end [1:] , and i following you need to slice the main list from leading to end except the last element , because the iterator will chose it :

>>> def pair(sample_list):
...    it_a=iter(sample_list[1:])
...    return [(i,it_a.next()) for i in sample_list[:-1]]
... 
>>> pair(a)
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6), (6, 7), (7, 8), (8, 9), (9, 10)]

Answer 5

Try this:

c = list(range(1, 11))
for a, b in zip(c[:-1], c[1:]):
    doSomething()