Modifying hash keys via the control variable

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

Dear monks, I have a HoH called %lists that looks like:

$VAR1 = {
          'outdoor' => {
                         'cat' => 'other',
                         'desc' => 'Outdoor activities'
                       },
          'Testlist' => {
                          'cat' => 'other',
                          'desc' => ''
                        },
}
[download]

I supposed that to modify the list (the hash's key), this should work:

for my $list ( keys %lists ) {
    $lists{$list}{desc} = check_desc $lists{$list}{desc};
    $list = "[mailto:$list\@example.com $list]";
}
[download]

The desc changes ok, but the list doesn't change. Why is that?

Well done is better than well said. -- Benjamin Franklin

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Re: Modifying hash keys via the control variable by choroba (Cardinal) on Dec 16, 2013 at 14:06 UTC
keys returns a list of strings, not aliases to the real hash keys. Imagine an array instead of a hash - would you expect the following to work? `# Simulate shift. (Does NOT work!) for my $i (1 .. $#array) { $i--; }` [download] To "change" a hash key, you have to delete it and create a new one: `$hash{newkey} = delete $hash{oldkey};` [download] لսႽ† ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l] [select]
Re: Modifying hash keys via the control variable by LanX (Saint) on Dec 16, 2013 at 14:05 UTC
Hash keys are immutable! You could delete the old one and create a new one. In your case better a complete new hash. But I'd rather add a new subkey email! Your data model attracts problems... Cheers Rolf ( addicted to the Perl Programming Language)	[reply]
Re: Modifying hash keys via the control variable by NetWallah (Canon) on Dec 16, 2013 at 15:03 UTC
You are better off creating a new hash, since both keys and values are being modified. `my %newhash = map { my $list=$_; "\[mailto:$list\@example.com $list]" => { map { $_ => $_ eq 'desc' ? check_desc $lists{$ +list}{desc} : $lists{$list}{$_} } keys %{$lists{$list} } keys %lists;` [download] Notes: 1 ) Untested 2) This is a Shallow copy of the inner hash. 3) '$list' is a confusing name for the key When in doubt, mumble; when in trouble, delegate; when in charge, ponder. -- James H. Boren	[reply] [d/l]