Haskell combine the elements of 2 lists at different index's

Question

Apologies for the awful title, I'm not too sure how to describe it in words but here is what I mean. If you know a better way to phrase this please let me know.

Suppose I had 2 lists, of equal length.

[a, b, c] [x, y, z]

I want to create the list

[[a, y, z], [b, x, z], [c, x, y]]

Essentially for every element of list1, I want the 2 elements at different indexes to the first element in list2.

so for "a" at index 0, the other 2 are "y" at index 1 and "z" at index 2.

I'm pretty sure I know how to do it using indexes, however, I know that that's not very efficient and wanted to see if there is a more functional solution out there.

Thank you.

question from:https://stackoverflow.com/questions/66050529/haskell-combine-the-elements-of-2-lists-at-different-indexs

Answer 1

You haven't described any edge cases, but you can get the basic behavior you're looking for with something like:

import Data.List (inits, tails)

combine :: [a] -> [a] -> [[a]]
combine xs ys = zipWith3 go xs (tails ys) (inits ys)
  where
    go a (_:xs) ys = a:ys ++ xs
    go _ _ _ = []

The key is that tails returns successive suffixes of its list starting with the full list, and inits returns successive prefixes starting with the empty list.