This is just a procrastination project for me, rather than anything that I expect to be useful. Here's the deal: check whether a number is the same when rotated half a turn (180 degrees or pi radians, whichever you prefer). (At this point, it's more a string than a number, but let's keep things simple.)

I've taken a few minor liberties with what counts as what when rotated; it's pretty easy to see that an upside-down 6 is a 9 (and vice versa), and that 1 and 8 are basically the same thing when rotated (despite the serifs on the 1 and any size difference your font might have on the 8's loops), but the 2<->5 thing is more of a judgement call.

Anyways, here's my implementation. It's a brute-force thing, lacking in both elegance and hack value. Can you come up with something cleverer? (This smells like a good problem to golf.)

#!/usr/bin/perl -w

use strict;

my %rotated = (
        0 => 0,
        1 => 1,
        2 => 2,
        3 => undef,
        4 => undef,
        5 => 5,
        6 => 9,
        7 => undef,
        8 => 8,
        9 => 6
    );

sub is_rotateable
{
    my ($n) = @_;

    my @num = split '', $n;
    my @mun = reverse @num;

    foreach (0..$#num) {
        my $d = $num[$_];
        return 0 if $rotated{$d} != $mun[$_];
    }

    return 1;
}

for (@ARGV) {
    print "$_ is rotateable\n" if &is_rotateable($_);
}
[download]

Edit: Yes, zeros are digits too. (D'oh!) Thanks Zaxo. Hmm... just offhand, I don't think this would be terribly interesting with, say, Roman numerals.

Edit: Duh. Thanks claree0... reading the replies to this post has been fun, but not exactly great for my ego. :-)

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