ch17_06.htm

by Randal L. Schwartz and Tom Phoenix ISBN 0-596-00132-0 Third Edition, published July 2001. (See

Actually, in a linguistic sense, it's more like a plural
marker, much like the letter "s" in words like
"cats" and "dogs." In Perl, the dollar sign
means there's just one of something, but the at-sign means
there's a list of items.
</p>

<p>A slice is always a list, so the array slice notation uses an at-sign
to indicate that. When you see something like <tt class="literal">@names[ ...
]</tt> in a Perl program, you'll need to do just as Perl
does and look at the at-sign at the beginning as well as the square
brackets at the end. The square brackets mean that you're
indexing into an array, and the at-sign means that you're
getting a whole list<a href="#FOOTNOTE-374">[374]</a>
of elements, not just a single one (which is what the dollar sign
would mean). See <a href="ch17_06.htm#lperl3-CHP-17-FIG-1">Figure 17-1</a>.
</p><blockquote class="footnote"> <a name="FOOTNOTE-374" /><p>[374]Of course, when we say "a
whole list," that doesn't necessarily mean more elements
than one -- the list could be empty, after all.</p> </blockquote>

<a name="lperl3-CHP-17-FIG-1" /><div class="figure"><img height="191" alt="Figure 17-1" src="figs/lrnp_1701.gif" width="161" /></div><h4 class="objtitle">Figure 17-1. Array slices versus single elements</h4>

<p>The punctuation mark at the front of the variable reference (either
the dollar sign or at-sign) determines the context of the subscript
expression. If there's a dollar sign in front, the subscript
expression is evaluated in a scalar context to get an index. But if
there's an at-sign in front, the subscript expression is
evaluated in a list context to get a list of indices.
</p>

<p>So we see that <tt class="literal">@names[ 2, 5 ]</tt> means the same list
as <tt class="literal">($names[2],</tt> <tt class="literal">$names[5])</tt> does.
If you want that list of values, you can simply use the array slice
notation. Any place you might want to write the list, you can instead
use the simpler array slice.
</p>

<p>But the slice can be used in one place where the list can't: a
slice may be interpolated directly into a string:
</p>

<blockquote><pre class="code">my @names = qw{ zero one two three four five six seven eight nine };
print "Bedrock @names[ 9, 0, 2, 1, 0 ]\n";</pre></blockquote>

<p>If we were to interpolate <tt class="literal">@names</tt>, that would give
all of the items from the array, separated by spaces. If instead we
interpolate <tt class="literal">@names[ 9, 0, 2, 1, 0 ]</tt>, that gives
just those items from the array, separated by spaces.<a href="#FOOTNOTE-375">[375]</a>
</p><blockquote class="footnote">
<a name="FOOTNOTE-375" /><p>[375]More accurately, the items of the list are separated by the
contents of Perl's <tt class="literal">$"</tt> variable, whose
default is a space. This should not normally be changed. When
interpolating a list of values, Perl internally does <tt class="literal">join
$", @list</tt>, where <tt class="literal">@list</tt> stands in for the
list expression. </p> </blockquote>

<p>Let's go back to the Bedrock Library for a moment. Maybe now
our program is updating Mr. Slate's address and phone number in
the patron file, because he just moved into a large new place in the
Hollyrock hills. If we've got a list of information about him
in <tt class="literal">@items</tt>, we could do something like this to
update just those two elements of the array:
</p>

<blockquote><pre class="code">my $new_home_phone = "555-6099";
my $new_address = "99380 Red Rock West";
@items[2, 3] = ($new_address, $new_home_phone);</pre></blockquote>

<p>Once again, the array slice makes a more compact notation for a list
of elements. In this case, that last line is the same as an
assignment to <tt class="literal">($items[2],</tt>
<tt class="literal">$items[3])</tt>, but more compact and efficient.
</p>

</div>
<a name="lperl3-CHP-17-SECT-6.2" /><div class="sect2">
<h3 class="sect2">17.6.2. Hash Slice</h3>

<p>In a way exactly analogous to an array slice, we can also slice some
elements from a <a name="INDEX-1125" />hash in a <em class="firstterm">hash
slice</em>. Remember when three of our characters went
bowling, and we kept their bowling scores in the
<tt class="literal">%score</tt> 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:
</p>

<blockquote><pre class="code">my @three_scores = ($score{"barney"}, $score{"fred"}, $score{"dino"});

my @three_scores = @score{ qw/ barney fred dino/ };</pre></blockquote>

<p>A slice is always a list, so the hash slice notation uses an at-sign
to indicate that.<a href="#FOOTNOTE-376">[376]</a> When you see something like <tt class="literal">@score{ ...
}</tt> in a Perl program, you'll need to do just as Perl
does and look at the at-sign at the beginning as well as the curly
braces at the end. The curly braces mean that you're indexing
into a hash; the at-sign means that you're getting a whole list
of elements, not just a single one (which is what the dollar sign
would mean). See <a href="ch17_06.htm#lperl3-CHP-17-FIG-2">Figure 17-2</a>.
</p><blockquote class="footnote"> <a name="FOOTNOTE-376" /><p>[376]If it sounds as if we're
repeating ourselves here, it's because we want to emphasize
that hash slices are analogous to array slices. If it sounds as if
we're not repeating ourselves here, it's because we want
to emphasize that hash slices are analogous to array slices. </p>
</blockquote>

<a name="lperl3-CHP-17-FIG-2" /><div class="figure"><img height="191" alt="Figure 17-2" src="figs/lrnp_1702.gif" width="150" /></div><h4 class="objtitle">Figure 17-2. Hash slices versus single elements</h4>

<p>As we saw with the array slice, the punctuation mark at the front of
the variable reference (either the dollar sign or at-sign) determines
the context of the subscript expression. If there's a dollar
sign in front, the subscript expression is evaluated in a scalar
context to get a single key.<a href="#FOOTNOTE-377">[377]</a> But if there's an at-sign in front, the
subscript expression is evaluated in a list context to get a list of
keys.
</p><blockquote class="footnote"> <a name="FOOTNOTE-377" /><p>[377]There's an
exception you're not likely to run across, since it isn't
used much in modern Perl code. See the entry for
<tt class="literal">$;</tt> in the <em class="emphasis">perlvar</em> manpage.
</p> </blockquote>

<p>It's normal at this point to wonder why there's no
percent sign ("<tt class="literal">%</tt>") here, when
we're talking about a hash. That's the marker that means
there's a whole hash; a hash slice (like any other slice) is
always a <em class="emphasis">list</em>, not a hash.<a href="#FOOTNOTE-378">[378]</a> In Perl, the dollar sign means
there's just one of something, but the at-sign means
there's a list of items, and the percent sign means
there's an entire hash.
</p><blockquote class="footnote"> <a name="FOOTNOTE-378" /><p>[378]A
hash slice is a slice (not a hash) in the same way that a house fire
is a fire (not a house), while a fire house is a house (not a fire).
More or less.</p> </blockquote>

<p>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:
</p>

<blockquote><pre class="code">my @players = qw/ barney fred dino /;
my @bowling_scores = (195, 205, 30);
@score{ @players } = @bowling_scores;</pre></blockquote>

<p>That last line does the same thing as if we had assigned to the
three-element list <tt class="literal">($score{"barney"}, $score{"fred"},
$score{"dino"})</tt>.
</p>

<p>A hash slice may be interpolated, too. Here, we print out the scores
for our favorite bowlers:
</p>

<a name="INDEX-1126" /><blockquote><pre class="code">print "Tonight's players were: @players\n";
print "Their scores were: @score{@players}\n";</pre></blockquote>

</div>


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