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 ```

Accepted Answer

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

```
def pairwise(iterable):
    "s -> (s0, s1), (s2, s3), (s4, s5), ..."
    a = iter(iterable)
    return zip(a, a)

for x, y in pairwise(l):
   print("%d + %d = %d" % (x, y, x + y))
```

Or, more generally:

```
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 zip(*[iter(iterable)]*n)

for x, y in grouped(l, 2):
   print("%d + %d = %d" % (x, y, x + y))
```

In Python 2, you should import `izip` as a replacement for Python 3's built-in `zip()` function.

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)
```

Iterating over every two elements in a list

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