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RPCFN #4 in Erlang

Posted on Fri, Mar 5, 2010 at 1:56 AM

A few weeks ago I wanted to start learning Erlang. A co-worker pointed out the Ruby Programming Challenge for Newbies that they were completing in Ruby. I decided to try the RPCFN #4, but write it in Erlang. This probably isn’t the most concise or best implementation, but it was a good exercise to encourage me to look at Erlang.

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%% This was inspired from the ruby programming challenge for newbies.
%%   http://rubylearning.com/blog/2009/11/26/rpcfn-rubyfun-4/



-module (polynomials).
%% -compile(export_all).
-export([poly_epr/1]).
-import(string, [concat/2]).
-import(lists, [append/1]).



%% include the test module
-include_lib("eunit/include/eunit.hrl").



%% Create a polynomial expression from an array of numbers
polyepr(List) when is_list(List), length(List) >= 2 ->
  P = gen_epr([], List),
  case R=join_epr(P) of
    "" ->
      "0";
     ->
      R
  end;
polyepr() ->
  {error, "Need at least 2 coefficients"}.



%% generate the polyonmial expression
genepr(Poly, []) ->
  case Poly of
    [] ->
      "0";
     ->
      Poly
  end;
gen_epr(Poly, [H|T]) ->
  Poly ++ gen_epr([term(H, length(T))], T).



%% join the expressions term together
join_epr([]) ->
  "0";
join_epr([H|T]) ->
  H ++ append([check_neg(X) || X <- T]).



%% add appropreiate sign in front of expression term
checkneg([]) ->
  "";
check_neg(Val="-" ++ T) ->
  Val;
check_neg(Val) ->
  concat("+",Val).



%% create an expression term
term(1, Expo) ->
  expo(Expo);
term(-1, Expo) ->
  "-" ++ expo(Expo);
term(0, Expo) ->
  "";
term(Val, 0) ->
  integer_to_list(Val);
term(Val, Expo) when is_number(Val), is_number(Expo)  -> 
  concat(integer_to_list(Val), expo(Expo));
term(Val, _Expo) ->
  "".



%% create the exponent expression
expo(1) -> 
  "x";
expo(E) ->
  concat("x^", integer_to_list(E)).



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% TESTS



%% term tests
term1_test() ->
  "x^2" = term(1,2).



term_negative_value_test() ->
  "-x^2" = term(-1,2).



term0_test() ->
  "" = term(0,5).



term_zero_exponent_test() ->
  "5" = term(5,0).



term_bad_values_test() ->
  "" = term("str","more").



%% poly tests from  the rpcfn
poly_epr1_test() ->
  ?assert("3x^3+4x^2-3" =:= poly_epr([3,4,0,-3])).  



poly_first_negative_test() ->
  ?assert("-3x^4-4x^3+x^2+6" =:= poly_epr([-3,-4,1,0,6])).



poly_simple_test() ->
  ?assert("x^2+2" =:= poly_epr([1,0,2])).



poly_first_minus_one_test() ->
  ?assert("-x^3-2x^2+3x" =:= poly_epr([-1,-2,3,0])).



poly_all_zera_test() ->
  ?assert("0" =:= poly_epr([0,0,0])).



poly_test_error_test() ->
  {error,Msg} = poly_epr([1]),
  ?assert("Need at least 2 coefficients" =:= Msg).

You need to have eunit setup in your code path. Then you can start the Erlang shell and run the tests. Really, they pass!

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$ erl
Erlang R13B03 (erts-5.7.4) [source] [smp:2:2] [rq:2] [async-threads:0] [kernel-poll:false]



Eshell V5.7.4  (abort with ^G)
1> c(polynomials).
{ok,polynomials}
2> polynomials:test().
  All 11 tests passed.
ok
3>

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