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12.16.6 中止生成式 — break和return</div>
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两个过程化语句可以用来提前中止一个生成式：<span style="color: #0000FF">break</span>和<span style="color: #0000FF">return</span>。这两个语句可以出现在任意代码块中；它们的不同之处在于它们退出的范围不同。 </p>
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<span style="color: #0000FF">break</span>语句中止序列的产生。当在一个生成式代码块中执行<span style="color: #0000FF">break</span>语句的时候，它会强制跳出<span style="color: #0000FF">randsequence</span>块。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00490"><pre class="Element12"><span style="color: #0000FF">randsequence</span><span style="color: #800080">()</span>
WRITE <span style="color: #800080">:</span> SETUP DATA<span style="color: #800080">;</span>
SETUP <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">if</span><span style="color: #800080">(</span>fifo_length<span style="color: #800080"> >= </span>max_length<span style="color: #800080">) </span><span style="color: #0000FF">break</span><span style="color: #800080">;}</span> COMMAND<span style="color: #800080">;</span>
DATA <span style="color: #800080">:</span> ...
<span style="color: #0000FF">endsequence</span>
next_statement <span style="color: #800080">:</span> ...</pre></div></div>
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当上面的例子在<span class="Element146">SETUP</span>生成式中执行<span style="color: #0000FF">break</span>语句的时候，不会产生<span class="Element146">COMMAND</span>生成式，并会继续执行标记为<span class="Element146">next_statement</span>的行。在一个循环语句中<span style="color: #0000FF">break</span>语句的使用与<a href="00592.html" target="topic">8.6节</a>中描述的方式相同。因此，<span style="color: #0000FF">break</span>语句中止最小包围的循环语句，否则是<span style="color: #0000FF">randsequence</span>块。 </p>
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<span style="color: #0000FF">return</span>语句中止生成当前的生成式。当在一个生成式代码块中执行一个<span style="color: #0000FF">return</span>语句的时候，会中止当前的生成式。序列产生会继续紧跟在被中止的生成式之后的下一个生成式。例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00491"><pre class="Element12"><span style="color: #0000FF">randsequence</span><span style="color: #800080">()</span>
TOP <span style="color: #800080">:</span> P1 P2<span style="color: #800080">;</span>
P1 <span style="color: #800080">:</span> A B C<span style="color: #800080">;</span>
P2 <span style="color: #800080">:</span> A <span style="color: #800080">{</span><span style="color: #0000FF">if</span><span style="color: #800080">(</span>flag<span style="color: #800080"> == </span><span style="color: #FF0000">1</span><span style="color: #800080">) </span><span style="color: #0000FF">return</span><span style="color: #800080">;}</span> B C<span style="color: #800080">;</span>
A <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"A"</span><span style="color: #800080">);};</span>
B <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">if</span><span style="color: #800080">(</span>flag<span style="color: #800080"> == </span><span style="color: #FF0000">2</span><span style="color: #800080">) return; </span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"B"</span><span style="color: #800080">);};</span>
C <span style="color: #800080">:</span> <span style="color: #800080">{</span><span style="color: #0000FF">$display</span><span style="color: #800080">(</span><span style="color: #575D94">"C"</span><span style="color: #800080">);};</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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根据<span class="Element146">flag</span>变量值的不同，上面的例子具有下列的结果： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00492"><pre class="Element12">flag == 0 ==> A B C A B C
flag == 1 ==> A B C A
flag == 2 ==> A C A C</pre></div></div>
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当<span class="Element146">flag<span style="color: #800080"> == </span><span style="color: #FF0000">1</span></span>的时候，生成式<span class="Element146">P2</span>会在产生<span class="Element146">A</span>之后中止。当<span class="Element146">flag<span style="color: #800080"> == </span><span style="color: #FF0000">2</span></span>的时候，生成式<span class="Element146">B</span>会中止两次（一次在<span class="Element146">P1</span>中，另外一次在<span class="Element146">P2</span>中），但每一次都会继续产生下一个生成式<span class="Element146">C</span>。</p></div>
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