204 lines
20 KiB
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204 lines
20 KiB
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<title>std::valarray<T>::operator[]</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::valarray<T>::</span>operator[]</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-numeric.html">numeric</a>‎ | <a href="cpp-numeric-valarray.html">valarray</a></span></div>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp">T operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> pos <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-until-cxx11">(C++11 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> pos <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp">T<span class="sy3">&</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> pos <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <a href="cpp-numeric-valarray-slice.html"><span class="kw1474">std::<span class="me2">slice</span></span></a> slicearr <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray-slice_array.html"><span class="kw1476">std::<span class="me2">slice_array</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <a href="cpp-numeric-valarray-slice.html"><span class="kw1474">std::<span class="me2">slice</span></span></a> slicearr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-numeric-valarray-gslice.html"><span class="kw1475">std::<span class="me2">gslice</span></span></a><span class="sy3">&</span> gslicearr <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray-gslice_array.html"><span class="kw1477">std::<span class="me2">gslice_array</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-numeric-valarray-gslice.html"><span class="kw1475">std::<span class="me2">gslice</span></span></a><span class="sy3">&</span> gslicearr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span><span class="sy3">&</span> boolarr <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray-mask_array.html"><span class="kw1478">std::<span class="me2">mask_array</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span><span class="kw4">bool</span><span class="sy1">></span><span class="sy3">&</span> boolarr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="sy3">&</span> indarr <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-valarray-indirect_array.html"><span class="kw1479">std::<span class="me2">indirect_array</span></span></a><span class="sy1"><</span>T<span class="sy1">></span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="sy3">&</span> indarr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p>获取数组的单个元素或一部分。</p>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 重载返回元素序列,并创建新的 <span class="t-lc"><a href="cpp-numeric-valarray.html">std::valarray</a></span> 对象。 非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 重载返回持有到数组元素引用的类。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td class="mbox-text" style="">本节未完成<br />
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原因:解释参数</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<div class="t-li1"><span class="t-li">1,2)</span> 到对应元素的引用</div>
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<div class="t-li1"><span class="t-li">3,5,7,9)</span> 含有被选择项副本的 <span class="t-lc"><a href="cpp-numeric-valarray.html">std::valarray</a></span> 对象</div>
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<div class="t-li1"><span class="t-li">4,6,8,10)</span> 包含到被选择项引用的对应数据结构</div>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>可能抛出实现定义的异常。</p>
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<h3><span class="mw-headline" id=".E5.89.8D.E6.9D.A1.E4.BB.B6">前条件</span></h3>
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<p>被选择元素必须存在。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<ul>
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<li>对于具体的值 <code><b>i</b></code> 与 <code><b>j</b></code> ,下列属性为 true :</li>
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</ul>
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<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>a<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1">=</span> q, a<span class="br0">[</span>i<span class="br0">]</span><span class="br0">)</span> <span class="sy1">==</span> q</span></span>
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<dd>对于非 const 的 <code><b>a</b></code> 。</dd>
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<div class="t-li1"><span class="t-li">2)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span>a<span class="br0">[</span>i<span class="sy2">+</span>j<span class="br0">]</span> <span class="sy1">==</span> <span class="sy3">&</span>a<span class="br0">[</span>i<span class="br0">]</span> <span class="sy2">+</span> j</span></span>
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<dd>这表示 valarray 元素在内存中毗邻。</dd>
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<div class="t-li1"><span class="t-li">3)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span>a<span class="br0">[</span>i<span class="br0">]</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="sy3">&</span>b<span class="br0">[</span>j<span class="br0">]</span></span></span>
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<dd>这对于每个互不为别称的对象 <code><b>a</b></code> 与 <code><b>b</b></code> 成立。</dd>
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<dd>这表示元素没有别称,而且此属性可用于进行某些类型的优化。</dd>
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</dl>
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<ul>
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<li>在 <a href="cpp-numeric-valarray-resize.html">resize</a> 或数组析构时引用变为非法。</li>
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</ul>
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<p>对于重载 <span class="t-v">(3,5,7,9)</span> ,函数能实现为拥有不同于 <span class="t-lc"><a href="cpp-numeric-valarray.html">std::valarray</a></span> 的返回类型。此情况下,替换类型拥有下列属性:</p>
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<dl>
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<dd>
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<ul>
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<li>提供 <span class="t-lc"><a href="cpp-numeric-valarray.html">std::valarray</a></span> 的所有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> 成员函数。</li>
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<li>能从替换类型构造 <span class="t-lc"><a href="cpp-numeric-valarray.html">std::valarray</a></span> 、 <span class="t-lc"><a href="cpp-numeric-valarray-slice_array.html">std::slice_array</a></span> 、 <span class="t-lc"><a href="cpp-numeric-valarray-gslice_array.html">std::gslice_array</a></span> 、 <span class="t-lc"><a href="cpp-numeric-valarray-mask_array.html">std::mask_array</a></span> 和 <span class="t-lc"><a href="cpp-numeric-valarray-indirect_array.html">std::indirect_array</a></span> 。</li>
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<li>所有接受一个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy3">&</span></span></span> 类型参数的函数<span class="t-rev-inl t-since-cxx11"><span>,除了 <a href="cpp-numeric-valarray-begin2.html"><tt>begin()</tt></a> 和 <a href="cpp-numeric-valarray-end2.html"><tt>end()</tt></a></span> <span><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></span></span>也应该接受替换类型。</li>
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<li>所有接受二个 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy3">&</span></span></span> 类型参数的函数都应该接受 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy3">&</span></span></span> 和替换类型的每种组合。</li>
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<li>返回类型添加不多于二层嵌套在最深层嵌套的参数类型上的模板。</li>
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</ul>
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</dd>
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<p>切片/掩码/间接访问不连锁: <span class="t-c"><span class="mw-geshi cpp source-cpp">v<span class="br0">[</span>v<span class="sy1">==</span>n<span class="br0">]</span><span class="br0">[</span><a href="cpp-numeric-valarray-slice.html"><span class="kw1474">std::<span class="me2">slice</span></span></a><span class="br0">(</span><span class="nu0">0</span>,<span class="nu0">5</span>,<span class="nu0">2</span><span class="br0">)</span><span class="br0">]</span> <span class="sy1">=</span> x<span class="sy4">;</span></span></span> 是错误,因为 <span class="t-lc"><a href="cpp-numeric-valarray-mask_array.html">std::mask_array</a></span> ( <span class="t-c"><span class="mw-geshi cpp source-cpp">v<span class="br0">[</span>v<span class="sy1">==</span>n<span class="br0">]</span></span></span> 的类型)无 <code>operator[]</code> 。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="co2">#include <valarray></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-numeric-valarray.html"><span class="kw1473">std::<span class="me2">valarray</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> data <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">0</span>,<span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span>,<span class="nu0">4</span>,<span class="nu0">5</span>,<span class="nu0">6</span>,<span class="nu0">7</span>,<span class="nu0">8</span>,<span class="nu0">9</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Initial valarray: "</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> n<span class="sy4">:</span> data<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> n <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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data<span class="br0">[</span>data <span class="sy1">></span> <span class="nu0">5</span><span class="br0">]</span> <span class="sy1">=</span> <span class="sy2">-</span><span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// operator[] 的 valarray<bool> 重载</span>
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<span class="co1">// data>5 的类型是 std::valarray<bool></span>
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<span class="co1">// data[data>5] 的类型是 std::mask_array<int></span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"After v[v>5]=-1: "</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> n <span class="sy1"><</span> data.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>n<span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> data<span class="br0">[</span>n<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span> <span class="co1">// 常规 operator[]</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出:</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Initial valarray: 0 1 2 3 4 5 6 7 8 9
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After v[v>5]=-1: 0 1 2 3 4 5 -1 -1 -1 -1</pre></div>
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