Truncatable primes: Difference between revisions

=={{header|Perl}}==

<lang perl>#!/usr/bin/perl

use warnings;

use strict;

use constant {

LEFT => 0,

RIGHT => 1,

};

{ my @primes = (2, 3);

sub is_prime {

my $n = shift;

return if $n < 2;

for my $prime (@primes) {

last if $prime >= $n;

return if 0 == $n % $prime;

}

my $sqrt = sqrt $n;

while ($primes[-1] < $sqrt) {

my $new = 2 + $primes[-1];

$new += 2 until is_prime($new);

push @primes, $new;

return if 0 == $n % $new;

}

return 1;

}

}

sub trunc {

my ($n, $side) = @_;

my $regex = $side == LEFT ? qr/^./ : qr/.$/;

$n =~ s/$regex//;

return $n;

}

sub is_tprime {

my ($n, $side) = @_;

return if $n =~ /0/;

return (is_prime($n) and (1 == length $n or is_tprime(trunc($n, $side), $side)));

}

my $length = 6;

my @tprimes = ('9' x $length) x 2;

for my $side (LEFT, RIGHT) {

$tprimes[$side] -= 2 until is_tprime($tprimes[$side], $side);

}

print 'left ', join(', right ', @tprimes), "\n";</lang>

{{out}}

<pre>left 998443, right 739399</pre>