My code for week 2 section 9 of FUNCTIONAL PROGRAMMING IN ERLANG THE UNIVERSITY OF KENT

two_nine.erl

-module (two_nine).
-export ([double/1,evens/1,median/1,modes/1]).

double([H]) -> [H * 2];
double([H|T]) ->
  [H * 2 | double(T)].

evens([H]) when 0 == H rem(2) -> [H];

evens([H|T]) ->
  case H of
    H when 0 == H rem(2) ->
      [H|evens(T)];
    _ ->
      evens(T)
  end.

median(List) ->
  % lists:sort(List) comes from the standard library
  median(lists:sort(List),length(List)).

median(List,Length) when 0 == Length rem(2) ->
  Split = Length div(2),
  (lists:nth(Split,List) + lists:nth(Split + 1,List))/2;
median(List,Length) ->
  lists:nth(Length div(2) + 1,List).

modes([H]) -> H;
modes(List) -> modes(lists:sort(List),1).

modes([H|T],Count) ->
  modes(T, H, Count, []).

modes([H],Last,Count,Accumulator) when H == Last ->
  display_mode(mode_accumulator(Accumulator,{Last,Count + 1}));

modes([_H],Last,Count,Accumulator) when Count > 1 ->
  display_mode(mode_accumulator(Accumulator,{Last,Count}));

modes([_H],_Last,_Count,Accumulator) ->
  display_mode(Accumulator);

modes([H|T],Last,Count,Accumulator) when H == Last ->
  modes(T,H,Count + 1, Accumulator);

modes([H|T],Last,Count,Accumulator) when Count > 1 ->
  modes(T,H,1, mode_accumulator(Accumulator,{Last,Count}));

modes([H|T],_Last,Count,Accumulator) ->
  modes(T,H,1, Accumulator).

mode_accumulator([],New) ->
  [New];
mode_accumulator([{_H,Hc}|_T]=Original,{_N,Nc}=New) ->
  case Original of
    Original when Hc > Nc ->
      Original;
    Original when Hc < Nc ->
      [New];
    _ ->
      lists:append(Original,New)
  end.

display_mode([{N,_Nc}]) ->
  N;
display_mode({N,_Nc}) ->
  N;
display_mode([{N,_Nc}|T]) ->
  [N, display_mode(T)].