Iterating over every two elements in a list [duplicate]
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, TypeVarT = 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.