string as a HASH ref

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

Why doesn't this work?

#!/usr/bin/perl5 -w
##########################################################
%fred = (1 => "un", 2 => "deux", 3 => "trois");
%mary = (1 => "eins", 2 => "zwei", 3 => "drei");

print "\nfred:"; 
for (keys %fred) {printf "\t%3s: %-8s", $_, $fred{$_}};
print "\nmary:"; 
for (keys %mary) {printf "\t%3s: %-8s", $_, $mary{$_}};

print "\n";
@nm = qw(fred mary);
for $n (@nm) {print "\n$n:";
for (keys %{$n}){printf "\t%3s: %-8s", $_, ${$n}{$_}}}

my %fred1 = (1 => "yksi", 2 => "kaksi", 3 => "kolme");
my %mary1 = (1 => "uno", 2 => "due", 3 => "tre");

use strict;

my @nm1 = qw(fred1 mary1);
print "\n";
for my $n (@nm1)
{
    print "\n$n:";
    for my $m (keys %{$n})
    {
        printf "\t%3s: %-8s", $m, ${$n}{$m};
    }
}
print "\n";

__END__

(svmoloch:szhxrv) $ ./test_hashname.pl

fred:     1: un           2: deux         3: trois   
mary:     1: eins         2: zwei         3: drei    

fred:     1: un           2: deux         3: trois   
mary:     1: eins         2: zwei         3: drei    

Can't use string ("fred1") as a HASH ref while "strict refs" in use at
+ ./test_hashname.pl line 27.
fred1:
[download]

Comment on string as a HASH ref Download Code

Replies are listed 'Best First'.
Re: string as a HASH ref by hardburn (Abbot) on Jun 18, 2003 at 14:22 UTC
Thou shalt not use symbolic refs unless thou has a really good reason. Here's why. One "really good reason" is for playing with the symbol table, which can be great fun and (among other things) makes generating almost-identical accessors and mutators for objects really easy. But for just accessing a variable, don't bother. ---- I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident. -- Schemer Note: All code is untested, unless otherwise stated	[reply]
Re: string as a HASH ref by tall_man (Parson) on Jun 18, 2003 at 14:22 UTC
You could make it work by putting a "no strict 'refs';" in front of it, but you shouldn't. Don't use variables as variable references. Instead, make a hash of hashes. `my %names; $names{fred1} = {1 => "un", 2 => "deux", 3 => "trois"}; $names{mary1} = {1 => "eins", 2 => "zwei", 3 => "drei"}; ... print $names{$n}{$m};` [download]	[reply] [d/l]
Re: string as a HASH ref by ant9000 (Monk) on Jun 18, 2003 at 14:17 UTC
Because of your 'use strict' pragma; try this instead: `for my $n (@nm1) { print "\n$n:"; no strict 'refs'; for my $m (keys %{$n}) { printf "\t%3s: %-8s", $m, ${$n}{$m}; } }` [download] and then take a look at perldoc perlref (search for strict).	[reply] [d/l]
Re: string as a HASH ref by Bilbo (Pilgrim) on Jun 18, 2003 at 14:25 UTC
Beacause this: `keys %{$n}` doesn't do what you think it does. This says 'list the keys in the hash which `$n` is a reference to', not 'list the keys in the hash of which `$n` is a string containing the name.' You almost certainly don't want to use variables to specify variable names - use a hash instead. Put all (both) your hashes into another hash. Ie: `my %numbers = {fred1 => \%fred1, mary1 => \%mary1};` [download] Try looking at perlref or this tutorial for examples of how to create and use complex data structures such as hashes of hashes. Also, why not put `use strict` at the top of the program. If it's worth using (which it is) why not use it on the whole program?	[reply] [d/l] [select]
Re: string as a HASH ref by GreyOwl (Initiate) on Jun 18, 2003 at 15:22 UTC
I am most impressed and humbly grateful for your prompt help! GreyOwl	[reply]