Incrementing characters

legLess has asked for the wisdom of the Perl Monks concerning the following question:

Monks ~

My CGI script is going to accept uploaded files and then put them somewhere. I don't want to overload one directory, so I'm using a three-level structure: "a/aa/aaa" to "z/zz/zzz". This way no directory has over 26 sub directories, no directory has more than $MAX (constant) files, and there are 26^3 possible directories.

my( $p1, $p2, $p3 ); # path parts, each one character, from database
my( $count ); # number of directory items, from database
if( $count > $DIR_MAX_ENTRIES ) {
    if( $p3 eq 'z' ) {
        $p3 = 'a';
        if( $p2 eq 'z' ) {
            $p2 = 'a';
            $p1 = chr( ord( $p1) + 1 );
        } else {
            $p2 = chr( ord( $p2) + 1 );
        }
    } else {
        $p3 = chr( ord( $p3) + 1 );
    }
}
[download]

It works as expected, with two caveats:

If the first character passes 'z' it barfs. This is fine: if we ever approach 'z' then the system will need redesign for other reasons.
'$count' may be susceptible to a race condition, since it's a stored number rather than an actual count of directory entries. This is fine: it's not meant to be high-precision, and $MAX will be low enough that some slop is ok.

Finally, though, I wonder if there's a faster or simpler way to do this. I'd thought about using an array of characters (rather than 'ord()' and 'chr()') and incrementing the subscript. This wouldn't eliminate the 'if eq z' check, though, just change its form and move it a bit.

Any advice from the monks? Thanks!
--
man with no legs, inc.

Comment on Incrementing characters Download Code

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Re: Incrementing characters by davorg (Chancellor) on Sep 06, 2001 at 20:15 UTC
Did you know that the increment operator (++) is magical when you pass it a string? Try this: `$a = 'a'; print $a++, "\n" for 1 .. 40;` [download] -- <http://www.dave.org.uk> Perl Training in the UK <http://www.iterative-software.com>	[reply] [d/l]
Re: Incrementing characters by bbfu (Curate) on Sep 06, 2001 at 20:16 UTC
`$var = "aaa"; ++$var; # or: $var++ print "Var is: $var\n"; __END__ Var is: aab` [download] Ah, the magic of Perl. Update: D'oh. davorg++ for speed. ;-) Update2: Oh, and just to make sure that it's clear: `($var = "az")++; $var == "ba";` bbfu Seasons don't fear The Reaper. Nor do the wind, the sun, and the rain. We can be like they are.	[reply] [d/l] [select]
Re: Incrementing characters by andye (Curate) on Sep 06, 2001 at 20:56 UTC
You could use a hash... `my %h=map{$_++ => $_}('a'..'z'); $h{'z'}='a';` [download] gives you a hash which looks like this: `a => b b => c c => d d => e e => f f => g g => h h => i i => j j => k k => l l => m m => n n => o o => p p => q q => r r => s s => t t => u u => v v => w w => x x => y y => z z => a` [download] andy.	[reply] [d/l] [select]
Re: Incrementing characters by George_Sherston (Vicar) on Sep 06, 2001 at 21:13 UTC
Do you insist that your sub-sub-directory name has three characters? If not, then `$s = $p1.$p2.$p3; $s++; $s =~ /(.*)(.{1})(.{1})/; ($p1,$p2,$p3) = ($1,$2,$3);` [download] Will generate an endless stream of path parts, with the added advantage you don't have to rewrite when you get to `z/z/z` because it goes straight on to `aa/a/a`. § George Sherston	[reply] [d/l] [select]
Re: Incrementing characters by John M. Dlugosz (Monsignor) on Sep 07, 2001 at 03:35 UTC
If you're using NTFS you don't have to worry about "overloading a directory". (overloading Explorer when you browse it, maybe...) but in my application I have thousands of files in one dir and it's simple and elegant.	[reply]