lib_misc.erl

-module(lib_misc).
-export([sum/1,for/3,qsort/1,pythag/1,perms/1]).
-export([odds_and_evens/1, odds_and_evens_acc/1]).

sum(L) -> sum(L, 0).

sum([], N)    -> N;
sum([H|T], N) -> sum(T, H+N).

for(Max, Max, F) -> [F(Max)];
for(I, Max, F)   -> [F(I)|for(I+1, Max, F)].

qsort([]) -> [];
qsort([Pivot | T]) ->
  qsort([X || X <- T, X < Pivot])
  ++ [Pivot] ++
  qsort([X || X <- T, X >= Pivot]).

pythag(N) ->
  [ {A,B,C} ||
      A <- lists:seq(1,N),
      B <- lists:seq(1,N),
      C <- lists:seq(1,N),
      A+B+C =< N,
      A*A+B*B =:= C*C].

perms([]) -> [[]];
perms(L) -> [[H|T] || H <- L, T <- perms(L--[H])].

odds_and_evens(L) ->
  Odds  = [ O || O <- L, (O rem 2) =:= 1 ],
  Evens = [ E || E <- L, (E rem 2) =:= 0 ],
  {Odds, Evens}.

odds_and_evens_acc(L) ->
  odds_and_evens_acc(L, [], []).

odds_and_evens_acc([H|T], Odds, Evens) ->
  case (H rem 2) of
    1 -> odds_and_evens_acc(T, [H|Odds], Evens);
    0 -> odds_and_evens_acc(T, Odds, [H|Evens])
  end;
odds_and_evens_acc([], Odds, Evens) ->
  {lists:reverse(Odds), lists:reverse(Evens)}.