On Mon, 2 May 2005, Paul Kraus wrote:
> Can someone for the sake of discussion give an example of why you
> would want to use a slice of a hash?
Quoting from _Learning Perl_:
In a way exactly analogous to an array slice, we can also slice
some elements from a hash in a hash slice. Remember when three of
our characters went bowling, and we kept their bowling scores in the
%score hash? We could pull those scores with a list of hash elements
or with a slice. These two techniques are equivalent, although the
second is more concise and efficient:
my @three_scores = ($score{"barney"}, $score{"fred"}, $score{"dino"});
my @three_scores = @score{ qw/ barney fred dino/ };
[...]
As we saw with array slices, a hash slice may be used instead of
the corresponding list of elements from the hash, anywhere within
Perl. So we can set our friends' bowling scores in the hash (without
disturbing any other elements in the hash) in this simple way:
my @players = qw/ barney fred dino /;
my @bowling_scores = (195, 205, 30);
@score{ @players } = @bowling_scores;
That last line does the same thing as if we had assigned to the
three-element list ($score{"barney"}, $score{"fred"}, $score{"dino"}).
A hash slice may be interpolated, too. Here, we print out the scores
for our favorite bowlers:
print "Tonight's players were: @players\n";
print "Their scores were: @[EMAIL PROTECTED]";
Quoting from the (*ahem*) upcoming _Perl Best Practices_:
[F]or accessing several elements of a hash: you change the leading $
of a regular hash access to @, then add as many keys as you like. So:
@active{'top', 'prev', 'backup'}
is exactly the same as:
($active{'top'}, $active{'prev'}, $active{'backup'})
The sliced version of the frames assignment will be marginally
faster than three separate scalar assignments, though the difference
in performance is probably not be significant unless you're doing
hundreds of millions of repetitions. The real benefit is in
comprehensibility and extensibility.
So, it's a lot like an array slice, and has many of the same advantages.
--
Chris Devers
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