fashimpaur
User
Feb 20, 2002, 6:54 AM
Post #2 of 3
(37959 views)
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Okay, as the forum states, today I give the answer and an explanation.
The answer is:
[perl]
%colors = reverse %fruits;
[/perl]
The explanation is quite simple. The perl function 'reverse'
expects one parameter, a LIST. If the method is invoked
where the return result expected is another LIST, the
value returned is a LIST in the opposite order. In this
case, since a hash is passed and it is a named
name/value pair list (as opposed to an anoymous LIST)
and we are expecting a list context (determined by the
left side of the equation), then an opposite ordered list is
stored in the %colors hash.
Because we said that the list we want to stuff our
returned value into is a special name/value pair list,
we can now use it's keys to get values returned.
Note that I specified that each value in the first hash had
to be unique. This is because when reverse occurs, had
two values been the same, the following scenario would
have occurred:
Assume that we had added $fruits{lemon} = yellow to
the hash.
reverse would process the first (no telling if it would be
banana or lemon) key 'yellow' the associated value
'lemon' (assuming that lemon was the first to process)
Then, later, when 'yellow' comes up again, the method
would see that there already was a key 'yellow' defined
and would set it's value to 'banana'. At this point, lemon
was lost. Hashes do not store the keys in any order. It is
purely random. Had the first processed been 'banana',
then $colors{yellow} would have the value 'lemon'
and 'banana' would be lost.
Then, you ask, why would you want to do this? Well, the
time you want to do this is when you have a known hash
than can only have unique values for the given keys.
Then, when it is reversed, you can reference a given
value to get it's key name. This might be useful for
figuring out what field name to store data in a database
after a form was submitted with only the value returned,
(like from a menu list on a cgi form).
Hope this was a useful note for you. If I have mistated
anything, I would appreciate a private reply so that I
may fix the error while this is editable.
Dennis
$a="c323745335d3221214b364d545".
"a362532582521254c3640504c3729".
"2f493759214b3635554c3040606a0",
print unpack"u*",pack "h*",$a,"\n\n";
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