challenge.erl

challenge.erl

-module(challenge).
-export([ swap/3 ]).

% no effect on an empty list
swap([ ], _, _) -> [ ];

% no effect on a list with a single item
swap([ H | [] ], _, _) -> [ H ];

% no effect when both indexes are the same
swap(List, X, X) -> List;

swap([ H | T ], 0, IndexB) -> [ find(T, IndexB - 1) | replace(T, IndexB - 1, H) ];
swap([ H | T ], IndexA, 0) -> [ find(T, IndexA - 1) | replace(T, IndexA - 1, H) ];
swap([ H | T ], IndexA, IndexB) ->
  [ H | swap(T, IndexA - 1, IndexB -1) ].

% returns an element from a a list at Index
find([ ], _) -> error(badIndex);
find([ H | _ ], 0) -> H;
find([ _ | T ], Index) -> find(T, Index - 1).

% returns a list with the element at Index replaced by New
replace([ ], _, _) -> error(badIndex);
replace([ _ | T ], 0, New) -> [ New | T ];
replace([ H | T ], Index, New) ->  [ H | replace(T, Index - 1, New) ].

Elixir -> https://gist.github.com/tdantas/417d172ad8c10d5cec7a