Variable scoping, foreach loop, alias

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

I'm not exactly new to perl and I'm not exactly a guru but for the life of me I can't figure out why this does what it does.

use strict;
my @arry = (1,2,3,4,5 );

foreach my $bla ( @arry )
{
    $bla++;
}

foreach ( @arry )
{
    print "$_\n";
}
[download]

Everything I've ever seen/done/read in perl has not
clarified why this thing prints
2
3
4
5
6

??? Things like this just make you feel dumb.
Oh there is one other thing, this script is running under
the nebulous environ of mod_perl if that makes a diff.

edit by thelenm: added code tags

_{janitored by ybiC: Retitle from less-than-descriptive "Okay, I'm needing some clarification."}

Comment on Variable scoping, foreach loop, alias Download Code

Replies are listed 'Best First'.
Re: Variable scoping, foreach loop, alias by Zaxo (Archbishop) on Jan 23, 2004 at 21:11 UTC
Your $bla is not an ordinary variable in the foreach loop. `foreach my $bla (@arry) {}` makes $bla an alias to each element of @arry in turn. The increment acts on the original array elements. After Compline, Zaxo	[reply] [d/l]
Re: Variable scoping, foreach loop, alias by thelenm (Vicar) on Jan 23, 2004 at 21:11 UTC
In your loop, `$bla` is being aliased to each element of the array in turn. So, when you modify `$bla`, you are really modifying the array element that `$bla` is referring to. The same thing happens with `$_` if you don't specify a variable. For example, try this as well: `my @arry = (1, 2, 3, 4); ++$_ for @arry; print "$_\n" for @arry;` [download] -- Mike -- `XML::Simpler does not require XML::Parser or a SAX parser. It does require File::Slurp. -- grantm, perldoc XML::Simpler`	[reply] [d/l] [select]
Re: (2) Variable scoping, foreach loop, alias by Paulster2 (Priest) on Jan 23, 2004 at 21:18 UTC
I didn't realize that it only became an alias. This explains a lot to me. I appreciate your two replies!! Thanx a lot zaxo and thelenm for the insight! Paulster2	[reply]
Re: Variable scoping, foreach loop, alias by Fletch (Bishop) on Jan 23, 2004 at 21:25 UTC
Read more, specifically `perldoc perlsyn`: If any element of LIST is an lvalue, you can modify it by modifying VAR inside the loop. Conversely, if any element if LIST is NOT an lvalue, any attempt to modify that element will fail. In other words, the "foreach" loop index variable is an implicit alias for each item in the list that you're looping over.	[reply] [d/l]
Re: Variable scoping, foreach loop, alias by Roy Johnson (Monsignor) on Jan 23, 2004 at 21:13 UTC
In a word, aliasing. Foreach loop variables are aliased to the elements they are iterating over. Modifying the loop variable modifies the original element. If you hard-code your list into the loop, instead of using an array, you'll get an error when it tries to modify it. The PerlMonk `tr///` Advocate	[reply]


The stupid question is the question not asked
	PerlMonks