Modifying the keys in a hash

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

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Re: Modifying the keys in a hash by Sidhekin (Priest) on Feb 23, 2007 at 10:10 UTC
No other way. Sorry. However, you can make it less clumsy (at least avoid repeating `$hash{'key1'}`) by using the return value from delete: `$hash{'key2'} = delete $hash{'key1'};` [download] `print "Just another Perl ${\(trickster and hacker)},"` The Sidhekin proves Sidhe did it!	[reply] [d/l] [select]
Re^2: Modifying the keys in a hash by Mandrake (Chaplain) on Feb 23, 2007 at 11:24 UTC
Thanks for the reply.	[reply]
Re: Modifying the keys in a hash by sluap (Sexton) on Feb 23, 2007 at 14:22 UTC
Hi, You could treat the hash as an array and modify the key with something like: `%hash = map { s/^key1$/new_key2/; $_; } %hash;` [download] However, I'm not sure I would recommend it, as I don't think it makes what you are trying to do any clearer, and it has the potential to modify values that also match the regex!	[reply] [d/l]
Re^2: Modifying the keys in a hash by merlyn (Sage) on Feb 23, 2007 at 15:12 UTC
`... map { s/^key1$/new_key2/; $_; } ...` [download] In general, this idiom is a bad idea, because the value of $_ is a reference to the original data. So not only do you have a new value on your output, you have also edited the values on the input (if possible)! If you want to make changes like that, make a local copy of $_: `... map { local $_ = $_; s/^key1$/new_key2/; $_; } ...` [download] -- Randal L. Schwartz, Perl hacker	[reply] [d/l] [select]
Re^3: Modifying the keys in a hash by sluap (Sexton) on Feb 23, 2007 at 15:44 UTC
That's interesting - I wondered if that could be used to modify the hash key without creating a copy of the whole hash so tried: `use Data::Dump qw(dump); my %hash = ( key1 => 'Value1', key2 => 'Value2', key3 => 'Value3', ); print "old hash is " . dump(\%hash) ."\n"; map { s/2/_two/; } %hash; print "new hash is " . dump(\%hash) ."\n"; # output is: # old hash is { key1 => "Value1", key2 => "Value2", key3 => "Value3" } # new hash is { key1 => "Value1", key2 => "Value_two", key3 => "Value3 +" }` [download] How come 'Value2' is changed to 'Value_two', but 'key2' is unchanged? (Note: I wasn't suggesting that this method should be used for modifying a hash key, but was trying to rise to the challenge of suggesting another method other than copy and delete!)	[reply] [d/l]
Re^4: Modifying the keys in a hash by shmem (Chancellor) on Feb 23, 2007 at 16:58 UTC
Re^2: Modifying the keys in a hash by shmem (Chancellor) on Feb 23, 2007 at 14:37 UTC
You could treat the hash as an array and modify the key Sure you could, but why? Re-construct the entire hash to modify just one key? That's feasible, but only for small hashes, since you are duplicating all keys and values of the hash into a temporary list. Pretty much overhead. --shmem _($_=" "x(1<<5)."?\n".q·/)Oo. G°\ / /\_¯/(q / ---------------------------- \__(m.====·.(_("always off the crowd"))."· ");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}	[reply]