Iterating over every two elements in a list

Question

How do I make a for loop or a list comprehension so that every iteration gives me two elements?

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

for i,k in ???:
    print str(i), '+', str(k), '=', str(i+k)

Output:

1+2=3
3+4=7
5+6=11

Answer 1

Using typing so you can verify data using mypy static analysis tool:

from typing import Iterator, Any, Iterable, TypeVar, Tuple

T_ = TypeVar('T_')
Pairs_Iter = Iterator[Tuple[T_, T_]]

def legs(iterable: Iterator[T_]) -> Pairs_Iter:
    begin = next(iterable)
    for end in iterable:
        yield begin, end
        begin = end

Answer 2

Here we can have alt_elem method which can fit in your for loop.

def alt_elem(list, index=2):
    for i, elem in enumerate(list, start=1):
        if not i % index:
           yield tuple(list[i-index:i])


a = range(10)
for index in [2, 3, 4]:
    print("With index: {0}".format(index))
    for i in alt_elem(a, index):
       print(i)

Output:

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

Note: Above solution might not be efficient considering operations performed in func.

Answer 3

While all the answers using zip are correct, I find that implementing the functionality yourself leads to more readable code:

def pairwise(it):
    it = iter(it)
    while True:
        yield next(it), next(it)

The it = iter(it) part ensures that it is actually an iterator, not just an iterable. If it already is an iterator, this line is a no-op.

Usage:

for a, b in pairwise([0, 1, 2, 3, 4, 5]):
    print(a + b)

Answer 4

Apologies for being late.I hope this will be even more elegant way of doing it.

a = [1,2,3,4,5,6]
zip(a[::2], a[1::2])

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

Answer 5

The title of this question is misleading, you seem to be looking for consecutive pairs, but if you want to iterate over the set of all possible pairs than this will work :

for i,v in enumerate(items[:-1]):
        for u in items[i+1:]:

Answer 6

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

>>> zip(l,l[1:])
[(1, 2), (2, 3), (3, 4), (4, 5), (5, 6)]

>>> zip(l,l[1:])[::2]
[(1, 2), (3, 4), (5, 6)]

>>> [a+b for a,b in zip(l,l[1:])[::2]]
[3, 7, 11]

>>> ["%d + %d = %d" % (a,b,a+b) for a,b in zip(l,l[1:])[::2]]
['1 + 2 = 3', '3 + 4 = 7', '5 + 6 = 11']