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Problem 8 is another one of those problems that’s pretty simple to approach. The only even minor twist is that it’s probably best to use a string in this one. But easily enough done.

Some on the P.E. discussion boards even did this one with no programming, just a visual scan of the number.

“Find the greatest product of five consecutive digits in the 1000-digit number.”

Strung out code -

 -module(euler8).
 -export([greatestproduct/0]).

greatestproduct() ->
   Source  = "731671765313306 [etc, really long number] 52963450",
   N = 1,
   InitialGreatest = 0,
   Greatest = teststring(Source,N, InitialGreatest),
   Greatest.

teststring(Source, N,Greatest) ->
   if N < (length(Source)-5) ->
         Anum = list_to_integer(string:substr(Source,N,1)),
         Bnum = list_to_integer(string:substr(Source,N+1,1)),
         Cnum = list_to_integer(string:substr(Source,N+2,1)),
         Dnum = list_to_integer(string:substr(Source,N+3,1)),
         Enum = list_to_integer(string:substr(Source,N+4,1)),
         Product = Anum*Bnum*Cnum*Dnum*Enum,
         if Product > Greatest ->
            teststring(Source, N+1, Product);
         true ->
            teststring(Source, N+1, Greatest)
         end;
   true ->
         Greatest
   end.