Question

I’m in the process of learning Erlang. As an exercise I picked up the Sieve of Eratosthenes algorithm of generating prime numbers. Here is my code:

-module(seed2). -export([get/1]).  get(N) -> WorkList = lists:duplicate(N, empty),           get(2, N, WorkList, []).  get(thats_the_end, _N, _WorkList, ResultList) -> lists:reverse(ResultList); get(CurrentPrime, N, WorkList, ResultList) -> ModWorkList = markAsPrime(CurrentPrime, N, WorkList),                                               NextPrime = findNextPrime(CurrentPrime + 1, N, WorkList),                                               get(NextPrime, N, ModWorkList, [CurrentPrime|ResultList]).   markAsPrime(CurrentPrime, N, WorkList) when CurrentPrime =< N -> WorkListMod = replace(CurrentPrime, WorkList, prime),                                                                  markAllMultiples(CurrentPrime, N, 2*CurrentPrime, WorkListMod).  markAllMultiples(_ThePrime, N, TheCurentMark, WorkList) when TheCurentMark > N -> WorkList; markAllMultiples(ThePrime, N, TheCurrentMark, WorkList) -> WorkListMod = replace(TheCurrentMark, WorkList, marked),                                                            markAllMultiples(ThePrime, N, TheCurrentMark + ThePrime, WorkListMod).  findNextPrime(Iterator, N, _WorkList) when Iterator > N -> thats_the_end; findNextPrime(Iterator, N, WorkList) -> I = lists:nth(Iterator, WorkList),                                         if                                           I =:= empty -> Iterator;                                           true -> findNextPrime(Iterator + 1, N, WorkList)                                         end.  replace(N, L, New)-> {L1, [_H|L2]} = lists:split(N - 1, L),                      lists:append(L1, [New|L2]). 

This code actually works 🙂 . The problem is that I have this feeling that it is not the best possible implementation.

My question is what would be the ‘erlangish’ way of implementing the ‘Sieve of Eratosthenes’

EDIT: OK, Andreas solution is very good but it is slow. Any ideas how to improve that?

Answer 1

Here’s a simple (but not terribly fast) sieve implementation:

-module(primes). -export([sieve/1]). -include_lib('eunit/include/eunit.hrl').  sieve([]) ->     []; sieve([H|T]) ->               List = lists:filter(fun(N) -> N rem H /= 0 end, T),     [H|sieve(List)]; sieve(N) ->     sieve(lists:seq(2,N)).