ch10_09.htm

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

<p>Here's a trick you might see, used to code up a nice multiway
branch:
</p>

<blockquote><pre class="code">my $size =
  ($width &lt; 10) ? "small"  :
  ($width &lt; 20) ? "medium" :
  ($width &lt; 50) ? "large"  :
                  "extra-large"; # default</pre></blockquote>

<p>That is really just three nested <tt class="literal">?:</tt> operators, and
it works quite well, once you get the hang of it.
</p>

<p>Of course, you're not obliged to use this operator. Beginners
may wish to avoid it. But you'll see it in others' code,
sooner or later, and we hope that one day you'll find a good
reason to use it in your own programs.
</p>

</div>
<a name="lperl3-CHP-10-SECT-9.3" /><div class="sect2">
<h3 class="sect2">10.9.3. Control Structures Using Partial-Evaluation Operators</h3>

<p><a name="INDEX-719" /> <a name="INDEX-720" />These three operators that we've
just seen -- <tt class="literal">&amp;&amp;</tt>, <tt class="literal">||</tt>,
and <tt class="literal">?:</tt> -- all share a peculiar property:
depending upon whether the value on the left side is true or false,
they may or may not evaluate an expression. Sometimes the expression
is evaluated, and sometimes it isn't. For that reason, these
are sometimes called <em class="firstterm">partial-evaluation</em>
operators, since they may not evaluate all of the expressions around
them. And partial-evaluation operators are automatically control
structures.<a href="#FOOTNOTE-238">[238]</a>
</p><blockquote class="footnote"> <a name="FOOTNOTE-238" /><p>[238]Some of you were wondering why these
logical operators are being covered in this chapter, weren't
you?</p> </blockquote>

<p>It's not as if Larry felt a burning need to add more control
structures to Perl. But once he had decided to put these
partial-evaluation operators into Perl, they automatically became
control structures as well. After all, anything that can activate and
deactivate a chunk of code is a control structure.
</p>

<p>Fortunately, you'll notice this only when the controlled
expression has side effects, like altering a variable's value
or causing some output. For example, suppose you ran across this line
of code:
</p>

<blockquote><pre class="code">($a &lt; $b) &amp;&amp; ($a = $b);</pre></blockquote>

<p>Right away, you should notice that the result of the logical AND
isn't being assigned anywhere.<a href="#FOOTNOTE-239">[239]</a> Why not?
</p><blockquote class="footnote"> <a name="FOOTNOTE-239" /><p>[239]But don't
forget to consider that it might be a return value, as the last
expression in a subroutine.</p> </blockquote>

<p>If <tt class="literal">$a</tt> is really less than <tt class="literal">$b</tt>,
the left side is true, so the right side will be evaluated, thereby
doing the assignment. But if <tt class="literal">$a</tt> is not less than
<tt class="literal">$b</tt>, the left side will be false, and thus the
right side would be skipped. So that line of code would do
essentially the same thing as this one, which is easier to
understand:
</p>

<blockquote><pre class="code">if ($a &lt; $b) { $a = $b; }</pre></blockquote>

<p>Or maybe you'll be maintaining a
<a name="INDEX-721" />program, and you'll see a line like
this one:
</p>

<blockquote><pre class="code">($a &gt; 10) || print "why is it not greater?\n";</pre></blockquote>

<p>If <tt class="literal">$a</tt> is really greater than ten, the left side is
true, and the logical OR is done. But if it's not, the left
side is false, and this will go on to print the message. Once again,
this could (and probably should) be written in the traditional way,
probably with <tt class="literal">if</tt> or <tt class="literal">unless</tt>.
</p>

<p>If you have a particularly twisted brain, you might even learn to
read these lines as if they were written in English. For example:
check that <tt class="literal">$a</tt> is less than <tt class="literal">$b</tt>,
<em class="emphasis">and if it is</em>, then do the assignment. Check that
<tt class="literal">$a</tt> is more than ten, <em class="emphasis">or if it's
not</em>, then print the message.
</p>

<p>It's generally former C programmers or old-time Perl
programmers who most often use these ways of writing control
structures. Why do they do it? Some have the mistaken idea that these
are more efficient. Some think these tricks make their code cooler.
Some are merely copying what they saw someone else do.
</p>

<p>In the same way, the ternary operator may be used for control. In
this case, we want to assign <tt class="literal">$c</tt> to the smaller of
two variables:
</p>

<blockquote><pre class="code">($a &lt; $b) ? ($a = $c) : ($b = $c);</pre></blockquote>

<p>If <tt class="literal">$a</tt> is smaller, it gets <tt class="literal">$c</tt>.
Otherwise, <tt class="literal">$b</tt> does.
</p>

<p>There is another way to write the logical
<a name="INDEX-722" />
<a name="INDEX-723" />AND and logical
<a name="INDEX-724" />OR
operators. You may wish to write them out as words:
<tt class="literal">and</tt> and <tt class="literal">or</tt>.<a href="#FOOTNOTE-240">[240]</a> These
word-operators have the same behaviors as the ones written with
punctuation, but the words are much lower on the precedence chart.
Since the words don't "stick" so tightly to the
nearby parts of the expression, they may need fewer parentheses:
</p><blockquote class="footnote">
<a name="FOOTNOTE-240" /><p>[240]There are also the low-precedence<a name="INDEX-725" /> <a name="INDEX-726" /> <tt class="literal">not</tt> (like the
logical-negation operator, "<tt class="literal">!</tt>") and
the rare <tt class="literal">xor</tt>.</p> </blockquote>

<blockquote><pre class="code">$a &lt; $b and $a = $b;  # but better written as the corresponding if</pre></blockquote>

<p>Then again, you may need <em class="emphasis">more</em> parentheses.
<a name="INDEX-727" />Precedence is a bugaboo. Be sure
to use parentheses to say what you mean, unless you're sure of
the precedence. Nevertheless, since the word forms are very low
precedence, you can generally understand that they cut the expression
into big pieces, doing everything on the left first, and then (if
needed) everything on the right.
</p>

<p>Despite the fact that using logical operators as control structures
can be confusing, sometimes they're the accepted way to write
code. We'll see a common use of the <tt class="literal">or</tt>
operator starting in the next chapter.
</p>

<p>So, using these operators as control structures is part of idiomatic
Perl -- Perl as she is spoken. Used properly, they can make your
code more powerful; otherwise they can make your code unmaintainable.
Don't overuse<a name="INDEX-728" /> <a name="INDEX-729" /> them.<a href="#FOOTNOTE-241">[241]</a><a name="INDEX-730" /> <a name="INDEX-731" /> 
</p><blockquote class="footnote">
<a name="FOOTNOTE-241" /><p>[241]Using these weird forms more than once per month counts as
overuse.</p> </blockquote>

</div>


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