Re: How do I find the key of a specific hash element?

On Mon, May 19, 2008 at 06:51:17PM -0500, Wagner, David --- Senior Programmer Analyst --- WGO wrote:
> > -----Original Message-----
> > From: jshock [mailto:[email protected]] 
> > Sent: Monday, May 19, 2008 07:20
> > To: [email protected]
> > Subject: How do I find the key of a specific hash element?
> > 
> > For example:
> > 
> > my %weekdays = (
> > 	0  => "SUN",
> > 	1 => "MON",
> > 	2 => "TUE",
> > 	3 => "WED",
> > 	4 => "THU",
> > 	5 => "FRI",
> > 	6 => "SAT",
> > );
> > $weekdays{2}; # gives "TUE"
> > 
> > But what if I know "TUE" and want to find out what the key is? Is
> > there a construct like $weekdays{"TUE"} that gives 2"
> > 
> 	It depends on what information you will have and how big and how
> often will you need to access that data. You can either search the
> weekdays hash looking for the value or build a hash that has the the
> keys turned around(ie, the SAT is the key and 6 is the value.

At some point as hash size increases it will be faster to use a reverse lookup
hash than doing a linear search through a hash converted to a list (I haven't
benchmarked it but the turn over point is probably for hashes with only 1 or 2
digits worth of keys).  However, I would generally always use the reverse hash
as I think it's cleaner looking code. Eg.

my %rev_weekdays = map { $weekdays{$_} => $_ } %weekdays;
print $rev_weekdays{TUE};

Cheers,

-J

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