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8.5.3 foreach循环</div>
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<span style="color: #0000FF">foreach</span>结构指定在数组元素上的迭代。它的自变量是一个指明任意类型数组（固定尺寸的、动态的、及联合数组）的标识符，然后紧跟着一个包围在方括号内的循环变量的列表。每一个循环变量对应于数组的某一维。<span style="color: #0000FF">foreach</span>结构类似于一个使用数组范围替代一个表达式来指定重复次数的<span style="color: #0000FF">repeat</span>循环。 </p>
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例子： </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00281"><pre class="Element12"><span style="color: #0000FF">string</span> words<span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">] = {</span><span style="color: #575D94">"hello"</span><span style="color: #800080">, </span><span style="color: #575D94">"world"</span><span style="color: #800080">};</span>
<span style="color: #0000FF">int</span> prod<span style="color: #800080">[</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">8</span><span style="color: #800080">][</span><span style="color: #FF0000">1</span><span style="color: #800080">:</span><span style="color: #FF0000">3</span><span style="color: #800080">];</span>
<span style="color: #0000FF">foreach </span><span style="color: #800080">(</span>words<span style="color: #800080">[</span>j<span style="color: #800080">])</span>
<span style="color: #0000FF">$display</span><span style="color: #800080">(</span>j<span style="color: #800080">,</span> words<span style="color: #800080">[</span>j<span style="color: #800080">]);</span> <span style="color: #008000">// 打印每一个索引和值</span>
<span style="color: #0000FF">foreach </span><span style="color: #800080">(</span>prod<span style="color: #800080">[</span>k<span style="color: #800080">,</span>m<span style="color: #800080">])</span>
prod<span style="color: #800080">[</span>k<span style="color: #800080">][</span>m<span style="color: #800080">] =</span> k <span style="color: #800080">*</span> m<span style="color: #800080">;</span> <span style="color: #008000">// 初始化</span></pre></div></div>
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循环变量的数目必须匹配数组变量的维数。空循环变量可以用来指示在对应的数组维数上没有迭代，并且趋向于尾部的连续空循环变量可以被忽略。循环变量是自动的、只读的，并且它们的作用范围对于循环来讲是本地的。每一个循环变量的类型被隐含地声明成与数组索引的类型一致。数组中任何具有相同标识符的循环变量都是错误的。 </p>
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循环变量到数组索引的映射由维基数确定，如<a href="00287.html" target="topic">23.7节</a>所描述的那样。对于<span style="color: #0000FF">foreach</span>循环，更高基数的索引变化也更快。 </p><div class="Element170">
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<div class="Element13"><div class="Element12" id="code00282"><pre class="Element12"><span style="color: #008000">// 1 2 3 3 4 1 2 -> 维数</span>
<span style="color: #0000FF">int</span> A <span style="color: #800080">[</span><span style="color: #FF0000">2</span><span style="color: #800080">][</span><span style="color: #FF0000">3</span><span style="color: #800080">][</span><span style="color: #FF0000">4</span><span style="color: #800080">];</span> <span style="color: #0000FF">bit</span> <span style="color: #800080">[</span><span style="color: #FF0000">3</span><span style="color: #800080">:</span><span style="color: #FF0000">0</span><span style="color: #800080">][</span><span style="color: #FF0000">2</span><span style="color: #800080">:</span><span style="color: #FF0000">1</span><span style="color: #800080">]</span> B <span style="color: #800080">[</span><span style="color: #FF0000">5</span><span style="color: #800080">:</span><span style="color: #FF0000">1</span><span style="color: #800080">][</span><span style="color: #FF0000">4</span><span style="color: #800080">];</span>
<span style="color: #0000FF">foreach</span><span style="color: #800080">(</span>A<span style="color: #800080">[</span>i<span style="color: #800080">,</span> j<span style="color: #800080">,</span> k<span style="color: #800080">])</span> ...
<span style="color: #0000FF">foreach</span><span style="color: #800080">(</span>B<span style="color: #800080">[</span>q<span style="color: #800080">,</span> r<span style="color: #800080">,</span> <span style="color: #800080">,</span> s<span style="color: #800080">])</span> ...</pre></div></div>
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对于第一条<span style="color: #0000FF">foreach</span>子句，<i>i</i>从<span style="color: #FF0000">0</span>到<span style="color: #FF0000">1</span>迭代，<i>j</i>从<span style="color: #FF0000">0</span>到<span style="color: #FF0000">2</span>迭代，<i>k</i>从<span style="color: #FF0000">0</span>到<span style="color: #FF0000">3</span>迭代。对第二条<span style="color: #0000FF">foreach</span>子句，<i>q</i>从<span style="color: #FF0000">5</span>到<span style="color: #FF0000">1</span>迭代，<i>r</i>从<span style="color: #FF0000">0</span>到<span style="color: #FF0000">3</span>迭代、<i>s</i>从<span style="color: #FF0000">2</span>到<span style="color: #FF0000">1</span>迭代（第三个索引的迭代被忽略）。 </p>
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多个循环变量对应于在指定索引上迭代的嵌套循环。循环的嵌套由维基数确定；外层循环对应于较低的基数索引。在上面的第一个例子中，最外层的循环在<i>i</i>上迭代，最内层的循环在<i>k</i>上迭代。 </p>
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当循环变量用在表达式当中而不是作为指定数组的索引的时候，它们被自动地强制转换到与索引类型一致的类型。对于固定尺寸或动态数组，强制转换类型时<span style="color: #0000FF">int</span>。对于以特定索引类型索引的联合数组，强制转换类型与索引类型一致。对于使用通配符索引（<span style="color: #FF0000">*</span>）进行索引的联合数组，强制转换类型是无符号的<span style="color: #0000FF">longint</span>类型。为了使用不同的类型，可以使用一个显式的强制类型转换。</p></div>
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