Part 2 of my submission for the shapes portion of the 1st assignment of FUNCTIONAL PROGRAMMING IN ERLANG THE UNIVERSITY OF KENT

bits.erl

-module(bits).
-export([bits/1,tests/0]).

bits(N) when N >= 0 ->
  % convert N into a base 2 list then back into a base 10 number
  bits(list_to_integer(integer_to_list(N,2)),0).

bits(0, Acc) -> Acc;

bits(N,Acc) ->
  % divide N by 10 to pass through to the recursive function
  % divide N by 2 to determine if 1 or 0 and add to the Accumulator
  bits(trunc(N/10), Acc + (N rem 2)).

tests() ->
  % test cases {Number to convrt, expected total}
  run_cases([{0,0},{1,1},{2,1},{3,2},{4,1},{5,2},{6,2},{7,3},{8,1},{9,2},{10,2}]).

run_cases([]) ->
  complete;

run_cases([H|T]) ->
  test(H),
  run_cases(T).

test({N,Acc}) ->
  Spec = (Acc == bits(N)),
  test_output_formatter(N, Spec).

test_output_formatter(Name, Spec) ->
  io:format(integer_to_list(Name) ++ ": " ++ atom_to_list(Spec) ++ "\n").