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Project Euler Problem 4

on project euler, erlang, perl, python

Largest palindrome product

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.
Link to original description
Source code examples on Github

Erlang version 1

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#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname p4
% vim:syn=erlang

-mode(compile).

main(_) ->
    Answer = lists:max([ X*Y || X <- lists:seq(999,100,-1), Y <- lists:seq(999,100,-1), is_palindrome(X*Y)]),
    io:format("Answer ~p ~n",[ Answer ]).


is_palindrome(N) -> S = integer_to_list(N), S =:= lists:reverse(S).

Erlang version 2

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#!/usr/bin/env escript
%% -*- erlang -*-
%%! -smp enable -sname p4
% vim:syn=erlang

-mode(compile).

main(_) ->
    Answer = lists:max([ p(X,X) || X <- lists:seq(999,100,-1)]),
    io:format("Answer ~p ~n",[ Answer ]).


p(X,Y)  when Y > 99  ->
    P = X*Y,
    case is_palindrome(P) of
        true -> P
        ;_   -> p(X,Y-1)
    end;
p(_,_) -> 0.

is_palindrome(N) -> S = integer_to_list(N), S =:= lists:reverse(S).

Perl version

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#!/usr/bin/perl -w
use strict;

my $answer = 0;
for my $x (-999..-100){
    for my $y ($x..-100){
        my $n = $x*$y;
        $answer = $n if $n > $answer && $n == reverse(split(//,$n))   
    }    
}
print "Answer: $answer \n";

Python version 1

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#!/usr/bin/python

n = 0
for a in xrange(999, 100, -1):
     for b in xrange(a, 100, -1):
         x = a * b
         if x > n:
             s = str(x)
             if s == s[::-1]: n = x
print n