# Iterating over every two elements in a list

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Solving problem is about exposing yourself to as many situations as possible like Iterating over every two elements in a list and practice these strategies over and over. With time, it becomes second nature and a natural way you approach any problems in general. Big or small, always start with a plan, use other strategies mentioned here till you are confident and ready to code the solution.
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Iterating over every two elements in a list

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
``````

You need a `pairwise()` (or `grouped()`) implementation.

For Python 2:

``````from itertools import izip
def pairwise(iterable):
"s -> (s0, s1), (s2, s3), (s4, s5), ..."
a = iter(iterable)
return izip(a, a)
for x, y in pairwise(l):
print "%d + %d = %d" % (x, y, x + y)
``````

Or, more generally:

``````from itertools import izip
def grouped(iterable, n):
"s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), (s2n,s2n+1,s2n+2,...s3n-1), ..."
return izip(*[iter(iterable)]*n)
for x, y in grouped(l, 2):
print "%d + %d = %d" % (x, y, x + y)
``````

In Python 3, you can replace `izip` with the built-in `zip()` function, and drop the `import`.

All credit to martineau for his answer to my question, I have found this to be very efficient as it only iterates once over the list and does not create any unnecessary lists in the process.

N.B: This should not be confused with the `pairwise` recipe in Python’s own `itertools` documentation, which yields `s -> (s0, s1), (s1, s2), (s2, s3), ...`, as pointed out by @lazyr in the comments.

Little addition for those who would like to do type checking with mypy on Python 3:

``````from typing import Iterable, Tuple, TypeVar
T = TypeVar("T")
def grouped(iterable: Iterable[T], n=2) -> Iterable[Tuple[T, ...]]:
"""s -> (s0,s1,s2,...sn-1), (sn,sn+1,sn+2,...s2n-1), ..."""
return zip(*[iter(iterable)] * n)
``````

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.

``````>>> 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']
``````

A simple solution.

```l = [1, 2, 3, 4, 5, 6]
for i in range(0, len(l), 2):
print str(l[i]), '+', str(l[i + 1]), '=', str(l[i] + l[i + 1])
```

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:
try:
yield next(it), next(it)
except StopIteration:
# no more elements in the iterator
return
``````

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)
``````

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)]
``````

In case you’re interested in the performance, I did a small benchmark (using my library `simple_benchmark`) to compare the performance of the solutions and I included a function from one of my packages: `iteration_utilities.grouper`

``````from iteration_utilities import grouper
import matplotlib as mpl
from simple_benchmark import BenchmarkBuilder
bench = BenchmarkBuilder()
def Johnsyweb(l):
def pairwise(iterable):
"s -> (s0, s1), (s2, s3), (s4, s5), ..."
a = iter(iterable)
return zip(a, a)
for x, y in pairwise(l):
pass
def Margus(data):
for i, k in zip(data[0::2], data[1::2]):
pass
def pyanon(l):
list(zip(l,l[1:]))[::2]
for i in range(0, len(l), 2):
l[i], l[i+1]
def mic_e(it):
def pairwise(it):
it = iter(it)
while True:
try:
yield next(it), next(it)
except StopIteration:
return
for a, b in pairwise(it):
pass
def MSeifert(it):
for item1, item2 in grouper(it, 2):
pass
bench.use_random_lists_as_arguments(sizes=[2**i for i in range(1, 20)])
benchmark_result = bench.run()
mpl.rcParams['figure.figsize'] = (8, 10)
benchmark_result.plot_both(relative_to=MSeifert)
`````` So if you want the fastest solution without external dependencies you probably should just use the approach given by Johnysweb (at the time of writing it’s the most upvoted and accepted answer).

If you don’t mind the additional dependency then the `grouper` from `iteration_utilities` will probably be a bit faster.

Some of the approaches have some restrictions, that haven’t been discussed here.

For example a few solutions only work for sequences (that is lists, strings, etc.), for example Margus/pyanon/taskinoor solutions which uses indexing while other solutions work on any iterable (that is sequences and generators, iterators) like Johnysweb/mic_e/my solutions.

Then Johnysweb also provided a solution that works for other sizes than 2 while the other answers don’t (okay, the `iteration_utilities.grouper` also allows setting the number of elements to “group”).

Then there is also the question about what should happen if there is an odd number of elements in the list. Should the remaining item be dismissed? Should the list be padded to make it even sized? Should the remaining item be returned as single? The other answer don’t address this point directly, however if I haven’t overlooked anything they all follow the approach that the remaining item should be dismissed (except for taskinoors answer – that will actually raise an Exception).

With `grouper` you can decide what you want to do:

``````>>> from iteration_utilities import grouper
>>> list(grouper([1, 2, 3], 2))  # as single
[(1, 2), (3,)]
>>> list(grouper([1, 2, 3], 2, truncate=True))  # ignored
[(1, 2)]
>>> list(grouper([1, 2, 3], 2, fillvalue=None))  # padded
[(1, 2), (3, None)]
``````

Use the `zip` and `iter` commands together:

I find this solution using `iter` to be quite elegant:

``````it = iter(l)
list(zip(it, it))
# [(1, 2), (3, 4), (5, 6)]
``````

Which I found in the Python 3 zip documentation.

``````it = iter(l)
print(*(f'{u} + {v} = {u+v}' for u, v in zip(it, it)), sep='n')
# 1 + 2 = 3
# 3 + 4 = 7
# 5 + 6 = 11
``````

To generalise to `N` elements at a time:

``````N = 2
list(zip(*([iter(l)] * N)))
# [(1, 2), (3, 4), (5, 6)]
``````