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This is my first real attempt at a semi-useful program in Erlang. It is the Sieve of Eratosthenes, which is a prime number generator. The main function is primes() which accepts the number that you would like to count up to. Luckly, Erlang is a Functional language, so I was able to do some fun things. I had to modify the algorithm, as the original algorithm uses arrays, and arrays aren't really used in Erlang. I suppose this is probably due to the functional roots that Erlang has. Scheme in practice tends to stay away from vectors and uses lists more. I decided to use lists in Erlang, and was able to do some nice recursion, as is normal with Scheme. -module(sieve).
-export([primes/1]).
%% Removes the multiples of a number from a list
remove_multiples(_, []) -> [];
remove_multiples(N, [H|T]) when (H rem N == 0) ->
remove_multiples(N, T);
remove_multiples(N, [H|T]) ->
[H|remove_multiples(N, T)].
%% Generate primes by creating a list of numbers from 0 to N.
%% Then recurse through the list, eliminating multiples at each recursion.
primes(N) ->
primes(N, lists:seq(2,N)).
primes(N, [H|T]) ->
StopNumber = math:sqrt(N),
if
H < StopNumber
[H|primes(N,remove_multiples(H, T))]; %% Does this blow your mind?
true -> [H|T]
end.
I'm not experienced with Erlang yet, so my apologies if I have bad Erlang style. I wanted to put N < math:sqrt(N) in the guard for the function, but apparantly, you can't put function results in guards, so I had to store it in the StopNumber variable, and use an if statement.
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