Exercise 004

e004.erl

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+% Find the largest palindrome made from the product of two 3-digit numbers.
+
+% A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91  99.
+% Find the largest palindrome made from the product of two 3-digit numbers.
+
+-module(e004).
+-export([
+    main/1
+]).
+
+product(Min, Max) ->
+    product(Min, Max, Max, Max, 0).
+product(Min, _, N1, _, Product) when N1 < Min ->
+    Product;
+product(Min, Max, N1, N2, Product) when N2 < Min ->
+    product(Min, Max, N1 - 1, Max, Product);
+product(Min, Max, N1, N2, Product) ->
+    P = trunc(N1 * N2),
+    S = integer_to_list(trunc(N1 * N2)),
+    R = lists:reverse(S),
+    if
+        S == R, P > Product ->
+            product(Min, Max, N1, N2 - 1, P);
+        true ->
+            product(Min, Max, N1, N2 - 1, Product)
+    end.
+
+main(_) ->
+    io:format("Largest palindrome product of 2-digit numbers: ~w~n", [product(10, 99)]),
+    io:format("Largest palindrome product of 3-digit numbers: ~w~n", [product(100, 999)]).