How Can I Iterate Over Pairs (or Groups) of Elements in a Python List?

Iterating over Pairs of Elements in a List

When iterating over a list, we often encounter situations where we require access to two elements at a time. To address this, we can utilize the concept of pairwise iteration.

Python's built-in zip() function can assist us in this regard. The pairwise() or grouped() method consumes an iterable and yields successive two-element subsequences. Here's how we can implement it:

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

Using this function, let's consider the list [1, 2, 3, 4, 5, 6]. To demonstrate pairwise iteration:

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

This will produce the following output:

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

Alternatively, the grouped() function allows us to iterate over n elements at a time. Here's an implementation:

def grouped(iterable, n=2):
    "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 instance, with n set to 3, we can iterate over three elements at a time:

for x, y, z in grouped(l, 3):
   print("%d + %d + %d = %d" % (x, y, z, x + y + z))

This approach is efficient as it iterates over the list only once and eliminates the need for intermediate lists.

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