Path: utzoo!attcan!uunet!cs.utexas.edu!samsung!munnari.oz.au!frankland-river!pem
From: pem@frankland-river.aaii.oz.au (Paul E. Maisano)
Newsgroups: comp.lang.perl
Subject: Creating a reference to an array index
Message-ID: <1361@frankland-river.aaii.oz.au>
Date: 17 Mar 90 14:48:06 GMT
Organization: Australian AI Institute
Lines: 61

A short while ago I posted a request to see if anyone could think of
a way I could generate a reference to a particular index of an array.

For scalars it is easy; I have used code like the following to parse
command line args. The effect is: 'prog -p x y -u 1 2 -p z' results in
	$pvar == 'x y z ', $uvar == '1 2 '

while ($_ = shift) {
    /^-p$/ && (*vptr = *pvar, next);
    /^-u$/ && (*vptr = *uvar, next);
    $vptr .= "$_ ";
}

Anyway, if I have an array, say @ary = (1, 2, 3)
I want to be able to get a reference to an element of that array, so
that I can manipulate it independently of the array.

Randal mentioned that a foreach might work as in (something like):
	foreach $x ($ary[1]) {
	    # manipulate x and modify the array
	}
The foreach will only have this effect when an actual array is passed to the
foreach. So you can't really use this method.

I mentioned that a grep would work as in the following:
	grep( do {
		# manipulate $_ and modify the array
		},
	      $ary[1] );
This has the draw back of not letting you name the variable to something
meaningful.

So in my search for neatness (or any kludge that would work) I came up
with the following:
	
	grep(*vptr = *_, $ary[1])
	# manipulate $vptr and modify the array

It makes sense when you think about it. `_' contains the symbol table entry
for the element of the array which we then assign to `vptr'.
(Ok, ok, so I peeked at the source :-)

Now I am content and can sleep again at nights.
But I am too tired to work out how to generate "Just Another Perl Hacker,"
using it. Randal ? :-)

Here's a test fragment:

#!/usr/local/bin/perl 
@ary = (1, 2, 3);
grep(*vptr = *_, $ary[1]);
print "@ary\n";		# => 1 2 3
$vptr++;
print "@ary\n";		# => 1 3 3

------------------
Paul E. Maisano
Australian Artificial Intelligence Institute
1 Grattan St. Carlton, Vic. 3053, Australia
Ph: +613 663-7922  Fax: +613 663-7937
Email: pem@aaii.oz.au   UUCP: {uunet,mcsun,ukc,nttlab}!munnari!aaii.oz.au!pem