Iterating through a list in pairs

Sometimes, it is useful to be able to take a list of things, and traverse it in pairs.

For an example, if you had the list [1, 2, 3, 4], traversing it in pairs would result in the first iteration getting [1, 2], then [2, 3], and finally[3, 4].

One use case for traversing a list this way is to find the deltas between each pair of items in a list. Another is to check if a given list has already been sorted.

Listed below are ways to implement this in a few different languages:

Ruby

> (1..4).each_cons(2) { |a, b| p [a, b] }
[1, 2]
[2, 3]
[3, 4]
 => nil

Elixir

iex(1)> [1, 2, 3, 4] |> Enum.chunk_every(2, 1, :discard)
[[1, 2], [2, 3], [3, 4]]

Python

>>> items = [1, 2, 3, 4]
for a, b in zip(items, items[1:]):
   print [a, b]
[1, 2]
[2, 3]
[3, 4]

Javascript

> const R = require('ramda')
> R.aperture(2, [1, 2, 3, 4])
[[1, 2], [2, 3], [3, 4]]

Note: The above example requires the https://ramdajs.com/ library.