June 2018 Update

Task/Kaprekar-numbers/Erlang/kaprekar-numbers.erl

@@ -0,0 +1,27 @@
+-mode(compile).
+-import(lists, [seq/2]).
+
+kaprekar(1) -> true;
+kaprekar(N) when N < 1 -> false;
+kaprekar(N) ->
+    Sq = N*N,
+    if
+        (N rem 9) =/= (Sq rem 9) -> false;
+        true -> kaprekar(N, Sq, 10)
+    end.
+
+kaprekar(_, Sq,  M) when (Sq div M) =:= 0 -> false;
+kaprekar(N, Sq, M) ->
+    L = Sq div M,
+    R = Sq rem M,
+    if
+        R =/= 0 andalso (L + R) =:= N -> true;
+        true -> kaprekar(N, Sq, M * 10)
+    end.
+
+main(_) ->
+    Numbers = [N || N <- seq(1, 9999), kaprekar(N)],
+    io:format("The Kaprekar numbers < 10,000 are ~p~n", [Numbers]),
+
+    CountTo1e6 = length(Numbers) + length([N || N <- seq(10001, 999999), kaprekar(N)]),
+    io:format("There are ~p Kaprekar numbers < 1,000,000", [CountTo1e6]).