switch.html

perl教程

<p>In reality, Table 1 covers 31 alternatives, because only the equality and
intersection tests are commutative; in all other cases, the roles of
the <code>$s</code> and <code>$c</code> variables could be reversed to produce a
different test. For example, instead of testing a single hash for
the existence of a series of keys (<code>match if exists $s-&gt;{$c}</code>),
one could test for the existence of a single key in a series of hashes
(<code>match if exists $c-&gt;{$s}</code>).</p>
<p>As <a href="../lib/Pod/perltodo.html">the perltodo manpage</a> observes, a Perl case mechanism must support all these
&quot;ways to do it&quot;.</p>
<p>
</p>
<hr />
<h1><a name="description">DESCRIPTION</a></h1>
<p>The Switch.pm module implements a generalized case mechanism that covers
the numerous possible combinations of switch and case values described above.</p>
<p>The module augments the standard Perl syntax with two new control
statements: <code>switch</code> and <code>case</code>. The <code>switch</code> statement takes a
single scalar argument of any type, specified in parentheses.
<code>switch</code> stores this value as the
current switch value in a (localized) control variable.
The value is followed by a block which may contain one or more
Perl statements (including the <code>case</code> statement described below).
The block is unconditionally executed once the switch value has
been cached.</p>
<p>A <code>case</code> statement takes a single scalar argument (in mandatory
parentheses if it's a variable; otherwise the parens are optional) and
selects the appropriate type of matching between that argument and the
current switch value. The type of matching used is determined by the
respective types of the switch value and the <code>case</code> argument, as
specified in Table 1. If the match is successful, the mandatory
block associated with the <code>case</code> statement is executed.</p>
<p>In most other respects, the <code>case</code> statement is semantically identical
to an <code>if</code> statement. For example, it can be followed by an <code>else</code>
clause, and can be used as a postfix statement qualifier.</p>
<p>However, when a <code>case</code> block has been executed control is automatically
transferred to the statement after the immediately enclosing <code>switch</code>
block, rather than to the next statement within the block. In other
words, the success of any <code>case</code> statement prevents other cases in the
same scope from executing. But see <a href="#allowing_fallthrough">Allowing fall-through</a> below.</p>
<p>Together these two new statements provide a fully generalized case
mechanism:</p>
<pre>
        <span class="keyword">use</span> <span class="variable">Switch</span><span class="operator">;</span>
</pre>
<pre>
        <span class="comment"># AND LATER...</span>
</pre>
<pre>
        <span class="variable">%special</span> <span class="operator">=</span> <span class="operator">(</span> <span class="string">woohoo</span> <span class="operator">=&gt;</span> <span class="number">1</span><span class="operator">,</span>  <span class="variable">d'oh</span> <span class="operator">=&gt;</span> <span class="number">1</span> <span class="operator">);</span>
</pre>
<pre>
        <span class="keyword">while</span> <span class="operator">(&lt;&gt;)</span> <span class="operator">{</span>
            <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$_</span><span class="operator">)</span> <span class="operator">{</span>
</pre>
<pre>
                <span class="variable">case</span> <span class="operator">(</span><span class="variable">%special</span><span class="operator">)</span> <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"homer\n"</span><span class="operator">;</span> <span class="operator">}</span>      <span class="comment"># if $special{$_}</span>
                <span class="variable">case</span> <span class="operator">/</span><span class="variable">a</span><span class="keyword">-z</span><span class="operator">/</span><span class="variable">i</span>     <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"alpha\n"</span><span class="operator">;</span> <span class="operator">}</span>      <span class="comment"># if $_ =~ /a-z/i</span>
                <span class="variable">case</span> <span class="operator">[</span><span class="number">1</span><span class="operator">..</span><span class="number">9</span><span class="operator">]</span>     <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"small num\n"</span><span class="operator">;</span> <span class="operator">}</span>  <span class="comment"># if $_ in [1..9]</span>
</pre>
<pre>
                <span class="variable">case</span> <span class="operator">{</span> <span class="variable">$_</span><span class="operator">[</span><span class="number">0</span><span class="operator">]</span> <span class="operator">&gt;=</span> <span class="number">10</span> <span class="operator">}</span> <span class="operator">{</span>                    <span class="comment"># if $_ &gt;= 10</span>
                    <span class="keyword">my</span> <span class="variable">$age</span> <span class="operator">=</span> <span class="operator">&lt;&gt;;</span>
                    <span class="variable">switch</span> <span class="operator">(</span><span class="keyword">sub</span><span class="operator">{</span> <span class="variable">$_</span><span class="operator">[</span><span class="number">0</span><span class="operator">]</span> <span class="operator">&lt;</span> <span class="variable">$age</span> <span class="operator">}</span> <span class="operator">)</span> <span class="operator">{</span>
</pre>
<pre>
                        <span class="variable">case</span> <span class="number">20</span>  <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"teens\n"</span><span class="operator">;</span> <span class="operator">}</span>     <span class="comment"># if 20 &lt; $age</span>
                        <span class="variable">case</span> <span class="number">30</span>  <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"twenties\n"</span><span class="operator">;</span> <span class="operator">}</span>  <span class="comment"># if 30 &lt; $age</span>
                        <span class="keyword">else</span>     <span class="operator">{</span> <span class="keyword">print</span> <span class="string">"history\n"</span><span class="operator">;</span> <span class="operator">}</span>
                                            <span class="operator">}</span>
                                        <span class="operator">}</span>
</pre>
<pre>
                <span class="keyword">print</span> <span class="string">"must be punctuation\n"</span> <span class="variable">case</span> <span class="operator">/\</span><span class="variable">W</span><span class="operator">/;</span>  <span class="comment"># if $_ ~= /\W/</span>
                        <span class="operator">}</span>
</pre>
<p>Note that <code>switch</code>es can be nested within <code>case</code> (or any other) blocks,
and a series of <code>case</code> statements can try different types of matches
-- hash membership, pattern match, array intersection, simple equality,
etc. -- against the same switch value.</p>
<p>The use of intersection tests against an array reference is particularly
useful for aggregating integral cases:</p>
<pre>
        <span class="keyword">sub</span><span class="variable"> classify_digit
        </span><span class="operator">{</span>
                <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$_</span><span class="operator">[</span><span class="number">0</span><span class="operator">]</span><span class="operator">)</span> <span class="operator">{</span> <span class="variable">case</span> <span class="number">0</span>            <span class="operator">{</span> <span class="keyword">return</span> <span class="string">'zero'</span> <span class="operator">}</span>
                                 <span class="variable">case</span> <span class="operator">[</span><span class="number">2</span><span class="operator">,</span><span class="number">4</span><span class="operator">,</span><span class="number">6</span><span class="operator">,</span><span class="number">8</span><span class="operator">]</span>    <span class="operator">{</span> <span class="keyword">return</span> <span class="string">'even'</span> <span class="operator">}</span>
                                 <span class="variable">case</span> <span class="operator">[</span><span class="number">1</span><span class="operator">,</span><span class="number">3</span><span class="operator">,</span><span class="number">4</span><span class="operator">,</span><span class="number">7</span><span class="operator">,</span><span class="number">9</span><span class="operator">]</span>  <span class="operator">{</span> <span class="keyword">return</span> <span class="string">'odd'</span> <span class="operator">}</span>
                                 <span class="variable">case</span> <span class="operator">/</span><span class="operator">[</span><span class="variable">A</span><span class="operator">-</span><span class="variable">F</span><span class="operator">]</span><span class="operator">/</span><span class="variable">i</span>     <span class="operator">{</span> <span class="keyword">return</span> <span class="string">'hex'</span> <span class="operator">}</span>
                               <span class="operator">}</span>
        <span class="operator">}</span>
</pre>
<p>
</p>
<h2><a name="allowing_fallthrough">Allowing fall-through</a></h2>
<p>Fall-though (trying another case after one has already succeeded)
is usually a Bad Idea in a switch statement. However, this
is Perl, not a police state, so there <em>is</em> a way to do it, if you must.</p>
<p>If a <code>case</code> block executes an untargeted <a href="../lib/Pod/perlfunc.html#item_next"><code>next</code></a>, control is
immediately transferred to the statement <em>after</em> the <code>case</code> statement
(i.e. usually another case), rather than out of the surrounding
<code>switch</code> block.</p>
<p>For example:</p>
<pre>
        <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$val</span><span class="operator">)</span> <span class="operator">{</span>
                <span class="variable">case</span> <span class="number">1</span>      <span class="operator">{</span> <span class="variable">handle_num_1</span><span class="operator">();</span> <span class="keyword">next</span> <span class="operator">}</span>    <span class="comment"># and try next case...</span>
                <span class="variable">case</span> <span class="string">"1"</span>    <span class="operator">{</span> <span class="variable">handle_str_1</span><span class="operator">();</span> <span class="keyword">next</span> <span class="operator">}</span>    <span class="comment"># and try next case...</span>
                <span class="variable">case</span> <span class="operator">[</span><span class="number">0</span><span class="operator">..</span><span class="number">9</span><span class="operator">]</span> <span class="operator">{</span> <span class="variable">handle_num_any</span><span class="operator">();</span> <span class="operator">}</span>       <span class="comment"># and we're done</span>
                <span class="variable">case</span> <span class="operator">/\</span><span class="variable">d</span><span class="operator">/</span>   <span class="operator">{</span> <span class="variable">handle_dig_any</span><span class="operator">();</span> <span class="keyword">next</span> <span class="operator">}</span>  <span class="comment"># and try next case...</span>
                <span class="variable">case</span> <span class="operator">/.*</span><span class="regex">/   { handle_str_any(); next }  # and try next case...
        }
        </span>
</pre>
<p>If $val held the number <code>1</code>, the above <code>switch</code> block would call the
first three <code>handle_...</code> subroutines, jumping to the next case test
each time it encountered a <a href="../lib/Pod/perlfunc.html#item_next"><code>next</code></a>. After the thrid <code>case</code> block
was executed, control would jump to the end of the enclosing
<code>switch</code> block.</p>
<p>On the other hand, if $val held <code>10</code>, then only the last two <code>handle_...</code>
subroutines would be called.</p>
<p>Note that this mechanism allows the notion of <em>conditional fall-through</em>.
For example:</p>
<pre>
        <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$val</span><span class="operator">)</span> <span class="operator">{</span>
                <span class="variable">case</span> <span class="operator">[</span><span class="number">0</span><span class="operator">..</span><span class="number">9</span><span class="operator">]</span> <span class="operator">{</span> <span class="variable">handle_num_any</span><span class="operator">();</span> <span class="keyword">next</span> <span class="keyword">if</span> <span class="variable">$val</span> <span class="operator">&lt;</span> <span class="number">7</span><span class="operator">;</span> <span class="operator">}</span>
                <span class="variable">case</span> <span class="operator">/\</span><span class="variable">d</span><span class="operator">/</span>   <span class="operator">{</span> <span class="variable">handle_dig_any</span><span class="operator">();</span> <span class="operator">}</span>
        <span class="operator">}</span>
</pre>
<p>If an untargeted <a href="../lib/Pod/perlfunc.html#item_last"><code>last</code></a> statement is executed in a case block, this
immediately transfers control out of the enclosing <code>switch</code> block
(in other words, there is an implicit <a href="../lib/Pod/perlfunc.html#item_last"><code>last</code></a> at the end of each
normal <code>case</code> block). Thus the previous example could also have been
written:</p>
<pre>
        <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$val</span><span class="operator">)</span> <span class="operator">{</span>
                <span class="variable">case</span> <span class="operator">[</span><span class="number">0</span><span class="operator">..</span><span class="number">9</span><span class="operator">]</span> <span class="operator">{</span> <span class="variable">handle_num_any</span><span class="operator">();</span> <span class="keyword">last</span> <span class="keyword">if</span> <span class="variable">$val</span> <span class="operator">&gt;=</span> <span class="number">7</span><span class="operator">;</span> <span class="keyword">next</span><span class="operator">;</span> <span class="operator">}</span>
                <span class="variable">case</span> <span class="operator">/\</span><span class="variable">d</span><span class="operator">/</span>   <span class="operator">{</span> <span class="variable">handle_dig_any</span><span class="operator">();</span> <span class="operator">}</span>
        <span class="operator">}</span>
</pre>
<p>
</p>
<h2><a name="automating_fallthrough">Automating fall-through</a></h2>
<p>In situations where case fall-through should be the norm, rather than an
exception, an endless succession of terminal <a href="../lib/Pod/perlfunc.html#item_next"><code>next</code></a>s is tedious and ugly.
Hence, it is possible to reverse the default behaviour by specifying
the string &quot;fallthrough&quot; when importing the module. For example, the 
following code is equivalent to the first example in <a href="#allowing_fallthrough">Allowing fall-through</a>:</p>
<pre>
        <span class="keyword">use</span> <span class="variable">Switch</span> <span class="string">'fallthrough'</span><span class="operator">;</span>
</pre>
<pre>
        <span class="variable">switch</span> <span class="operator">(</span><span class="variable">$val</span><span class="operator">)</span> <span class="operator">{</span>