Aliasing one hash to another

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

Brethern,
I have a function, markFail, in some old code, which is hardcoded to writes value to a hash, %problemClassProps. In my new code, I'd like to write the same data to a different hash, %MEClassProps. Both hashes are global. I naively thought the following would work.

use strict;
use vars qw( %problemClassProps %MEClassProps );

sub assign_ME_problem{
    my($ontCategory, $ontNumbering, $mapwd) =@_;
    local(%problemClassProps) = %MEClassProps;
    markFail($mapwd, $ontCategory, $ontNumbering);
    1}
[download]

When I come back from markFail, the info is in %problemClassProps, not %MEClassProps, and I lose it when I exit the scope of assign_ME_problem. I want to (locally) alias %MEClassProps to %problemClassProps. I looked through data::alias figuring it would tell me how, but no joy. I want to leave the old code alone. What's the right way to approach this?

thanks,
throop

Comment on Aliasing one hash to another Download Code

Replies are listed 'Best First'.
Re: Aliasing one hash to another by imp (Priest) on Jan 30, 2007 at 22:56 UTC
You should just fix the offending code. Modifying a global hash is a bad practice. If for some reason this is not possible then you can use a glob to alias the second hash temporarily.. but this is bad mojo and should be avoided. `use strict; use warnings; use Data::Dumper; use vars qw( %hash1 %hash2 ); %hash1 = (a => 1); %hash2 = (b => 2); do_badness(); print "hash1: ", Dumper \%hash1; print "hash2: ", Dumper \%hash2; sub do_badness { local *hash1 = \%hash2; bad_sub(); } sub bad_sub { $hash1{c} = 3; }` [download]	[reply] [d/l]
Re: Aliasing one hash to another by Roy Johnson (Monsignor) on Jan 30, 2007 at 22:54 UTC
Aliasing is done with ~~symbolic references~~ globs. Try this: `local(problemClassProps) = \%MEClassProps;` [download] Update: terminology fixed. Caution:* Contents may have been coded under pressure.	[reply] [d/l]
Re: Aliasing one hash to another by Zaxo (Archbishop) on Jan 30, 2007 at 23:31 UTC
Roy Johnson has shown you the correct way to produce the alias. It's not really a symbolic reference, but a typeglob assignment. That is admittedly a thin distinction. I'll show in more detail how you would use this: `#!/usr/bin/perl use warnings; use strict; use vars qw/%MEClassProps/; use Old::Code qw/markFail/; # import the old function # . . . # time to use it, mostly your code sub assign_ME_problem{ my($ontCategory, $ontNumbering, $mapwd) =@_; local(%problemClassProps) = \%MEClassProps; markFail($mapwd, $ontCategory, $ontNumbering); 1; }` [download] You were on the right track, but just missed the use of a reference to alias the typeglob. After Compline, Zaxo	[reply] [d/l]
Re: Aliasing one hash to another by jdporter (Paladin) on Jan 30, 2007 at 23:55 UTC
There's a much simpler way to achieve this; though it does have a bit more overhead, it should only be significant for "large" hashes, since it involves copying all the elements. `sub assign_ME_problem { my( $ontCategory, $ontNumbering, $mapwd ) = @_; local( %problemClassProps ) = %problemClassProps; # localize a cop +y markFail( $mapwd, $ontCategory, $ontNumbering ); %MEClassProps = %problemClassProps; # copy to your hash 1 }` [download] A word spoken in Mind will reach its own level, in the objective world, by its own weight	[reply] [d/l]
Re: Aliasing one hash to another by xmath (Hermit) on Feb 11, 2007 at 12:16 UTC
Data::Alias should actually work fine... `alias local %problemClassProps = %MEClassProps;` This has the benefit of only localizing %problemClassProps, not all of *problemClassProps like Roy's solution.	[reply] [d/l]