Playing with $[

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

I've been trying to map out the rules that govern manipulation of $[. Basically what I want to do is something like:

@rray = ( 'foo', 'bar', 'spam', 'eggs' );
for ($calar=0; $calar < @rray; $[++) {
   print "$rray[$calar]";
}
[download]

but when I run the above code I get That use of $[ is unsupported at obf.pl line 2. The below code sort of works, but the value of $[ gets reset to 0 for every iteration so I get a console full of foo.

@rray = ( 'foo', 'bar', 'spam', 'eggs' );
for ($calar=0; $calar < @rray; $[ = $[+1) {
   print "$rray[$calar]";
}
[download]

Even if I increment another variable and assign its value to $[, I get the same That use of $[ is unsupported at obf.pl line 2. error as before. What is going on? How can I use $[ to freely play with arrays?

LAI
:eof

edited: Sat Jul 6 16:39:12 2002 by jeffa - title change (was: Playing with <code>$</code>?)

Comment on Playing with $[ Select or Download Code

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Re: Playing with <code>$[</code>? by Juerd (Abbot) on Jul 06, 2002 at 14:46 UTC
Why are your symbols called calar and rray? I think you thought `$[` is some array index pointer, but it's not. `$[` sets the base index, and you should never change it. Its documentation (from perlvar) clearly states what it does: `$[` The index of the first element in an array, and of the first character in a substring. Default is 0, but you could theoretically set it to 1 to make Perl behave more like awk (or Fortran) when subscripting and when evaluating the index() and substr() functions. (Mnemonic: `[` begins subscripts.) As of release 5 of Perl, assignment to `$[` is treated as a compiler directive, and cannot influence the behavior of any other file. Its use is highly discouraged. You probably meant something like: `@array = qw(foo bar spam eggs); for ($index = 0; $index < @array; $index++) { print $array[$index]; }` [download] But in that case, I think you should really use the simpler and more efficient `for VAR (LIST) BLOCK`. And don't forget to use strict. `use strict; my @array = qw(foo bar spam eggs); for my $element (@array) { print $element; }` [download] (Note that the last for loop could also just have been `print for @array`, or if `$,` isn't set even `print @array`.) - Yes, I reinvent wheels. - Spam: Visit eurotraQ.	[reply] [d/l] [select]
Re: Playing with $[ by cLive ;-) (Prior) on Jul 06, 2002 at 15:00 UTC
Good advice Juerd, but I think "obf.pl" is probably short for obfuscate.pl - hence the touching-things-that-should-be-left-alone-type-thingy... .02 cLive ;-) -- seek(JOB,$$LA,0);	[reply]
Re: Re: Playing with $[ by LAI (Hermit) on Jul 06, 2002 at 19:02 UTC
cLive ;-) is right. The whole point of this exercise is finding fun obfuscation methods. I read through the camel book on a regular basis, I know what the role of $[ is, I definitely know how to iterate through an array; I'm looking for fun and obscene ways to screw with code. LAI :eof	[reply]
Re: Playing with <code>$[</code>? by licking9Volts (Pilgrim) on Jul 06, 2002 at 15:45 UTC
Recently, I too had a question about the `$[` variable and got some great responses regarding its use. You might find them useful as well: Special variable - set array base.	[reply] [d/l]
Re: Playing with <code>$[</code>? by broquaint (Abbot) on Jul 06, 2002 at 14:53 UTC
I believe the use of `$[` has been deprecated in recent versions of perl which is why you're getting the error. You can achieve the same effect by taking advantage the `for` statement modifer `@array = qw(foo bar spam eggs); print for @array;` [download] This is infintely safer and saner than mucking about with `$[` who's inclusion is mainly for programmer compatibility with `AWK`. See the `perlvar` manpage for more info on `$[`. HTH `_________ broquaint`	[reply] [d/l] [select]
Re: Playing with $[ by jsprat (Curate) on Jul 06, 2002 at 18:29 UTC
The 'use' that perl is complaining about is '`$[ = $[+1`'. From perlvar, As of release 5 of Perl, assignment to $[ is treated as a compiler directive You can assign to it at compile time (ie `$[ = 5;` ), but cannot modify it at runtime (ie `$[++;` ).	[reply]
Re: Re: Playing with $[ by LAI (Hermit) on Jul 06, 2002 at 19:19 UTC
Okay, I get it now. So I could still do some playing around with `@rray = ( 'foo', 'bar', 'spam', 'eggs' ); $[=3; $calar = $[; print $rray[$calar] while $calar++<@rray;` [download] but not `@rray = ( 'foo', 'bar', 'spam', 'eggs' ); $[=$#rray $calar = $[; print $rray[$calar] while $calar++<@rray;` [download] Good to know. Thanks, jsprat LAI :eof	[reply] [d/l] [select]