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12.16.5 交叉生成式 — rand join</div>
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<span style="color: #0000FF">rand join</span>生成式控制被用来随机地交叉两个或多个生成式序列而又能维持每一个序列的相对顺序。<span style="color: #0000FF">rand join</span>生成式控制的语法如下： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00485"><pre class="Element12">rs_production_list ::=
rs_prod {rs_prod}
| <strong><span style="color: #FF0000">rand join</span></strong> [<strong><span style="color: #FF0000">(</span></strong>expression<strong><span style="color: #FF0000">)</span></strong>] production_item production_item {production_item}</pre></div></div>
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例如： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00486"><pre class="Element12"><span style="color: #0000FF">randsequence</span><span style="color: #800080">(</span>TOP<span style="color: #800080">)</span>
TOP <span style="color: #800080">:</span> <span style="color: #0000FF">rand join</span> S1 S2<span style="color: #800080">;</span>
S1 <span style="color: #800080">:</span> A B<span style="color: #800080">;</span>
S2 <span style="color: #800080">:</span> C D<span style="color: #800080">;</span>
<span style="color: #0000FF">endsequence</span></pre></div></div>
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这个发生器会随机地产生下列的序列： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00487"><pre class="Element12">A B C D
A C B D
A C D B
C D A B
C A B D
C A D B</pre></div></div>
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紧跟在<span style="color: #0000FF">rand join</span>关键字之后的可选的表达式必须是位于<span style="color: #FF0000">0.0</span>到<span style="color: #FF0000">1.0</span>范围内的一个实数。这个表达式的值代表了被交叉的序列长度影响一个序列被选择概率的程度。序列的长度是在一个给定的时间没有交叉的生成式的数目。如果表达式为<span style="color: #FF0000">0.0</span>，那么最短的序列具有较高的优先级；如果表达式为<span style="color: #FF0000">1.0</span>，那么最长的序列被指定了优先级。例如，使用前面的例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00488"><pre class="Element12">TOP <span style="color: #800080">:</span> <span style="color: #0000FF">rand join</span> <span style="color: #800080">(</span><span style="color: #FF0000">0.0</span><span style="color: #800080">)</span> S1 S2<span style="color: #800080">;</span></pre></div></div>
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对序列<span class="Element146">A B C D C D A B</span>给出了较高的优先级。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00489"><pre class="Element12">TOP <span style="color: #800080">:</span> <span style="color: #0000FF">rand join</span> <span style="color: #800080">(</span><span style="color: #FF0000">1.0</span><span style="color: #800080">)</span> S1 S2<span style="color: #800080">;</span></pre></div></div>
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给序列<span class="Element146">A C B D A C D B C A B D C A D B</span>赋予了较高的优先级。 </p>
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如果没有说明的话，发生器使用<span style="color: #FF0000">0.5</span>作为缺省值，它不会对任何序列长度区分优先级。 </p>
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在每一个步骤，发生器将非端接符号交叉成深度1。</p></div>
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