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

a_list = [1,2,3,4,5,6]
empty_list = [] 
for i in range(0,len(a_list),2):
   empty_list.append(a_list[i]+a_list[i+1])   
print(empty_list)

Answer 2

for (i, k) in zip(l[::2], l[1::2]):
    print i, "+", k, "=", i+k

zip(*iterable) returns a tuple with the next element of each iterable.

l[::2] returns the 1st, the 3rd, the 5th, etc. element of the list: the first colon indicates that the slice starts at the beginning because there's no number behind it, the second colon is only needed if you want a 'step in the slice' (in this case 2).

l[1::2] does the same thing but starts in the second element of the lists so it returns the 2nd, the 4th, 6th, etc. element of the original list.

Answer 3

you can use more_itertools package.

import more_itertools

lst = range(1, 7)
for i, j in more_itertools.chunked(lst, 2):
    print(f'{i} + {j} = {i+j}')

Answer 4

Thought that this is a good place to share my generalization of this for n>2, which is just a sliding window over an iterable:

def sliding_window(iterable, n):
    its = [ itertools.islice(iter, i, None) 
            for i, iter
            in enumerate(itertools.tee(iterable, n)) ]                               

    return itertools.izip(*its)

Answer 5

Well you need tuple of 2 elements, so

data = [1,2,3,4,5,6]
for i,k in zip(data[0::2], data[1::2]):
    print str(i), '+', str(k), '=', str(i+k)

Where:

• data[0::2] means create subset collection of elements that (index % 2 == 0)
• zip(x,y) creates a tuple collection from x and y collections same index elements.

Answer 6

For anyone it might help, here is a solution to a similar problem but with overlapping pairs (instead of mutually exclusive pairs).

From the Python itertools documentation:

from itertools import izip

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

Or, more generally:

from itertools import izip

def groupwise(iterable, n=2):
    "s -> (s0,s1,...,sn-1), (s1,s2,...,sn), (s2,s3,...,sn+1), ..."
    t = tee(iterable, n)
    for i in range(1, n):
        for j in range(0, i):
            next(t[i], None)
    return izip(*t)