162 lines
15 KiB
HTML
162 lines
15 KiB
HTML
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<title>std::sqrt(std::valarray)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_valarray_sqrt skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>sqrt<span style="font-size:0.7em; line-height:130%">(std::valarray)</span></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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<div>定义于头文件 <code><a href="cpp-header-valarray.html"><valarray></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> T <span class="sy1">></span><br />
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valarray<span class="sy1"><</span>T<span class="sy1">></span> sqrt<span class="br0">(</span> <span class="kw4">const</span> valarray<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> va <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td></td>
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<td></td>
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<p>对 <code>va</code> 中每个元素计算元素值的平方根。</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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<table class="t-par-begin">
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<td>va</td>
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<td>-</td>
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<td>要应用操作到的值数组</td>
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</tr>
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</table>
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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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<p>含有 <code>va</code> 中值的平方根的值数组。</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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<p>用无限定函数 (<span class="t-c"><span class="mw-geshi cpp source-cpp">sqrt</span></span>) 进行计算。若该函数不可用，则因参数依赖查找使用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-math-sqrt.html"><span class="kw1329">std::<span class="me2">sqrt</span></span></a></span></span> 。</p>
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<p>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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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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valarray<span class="sy1"><</span>T<span class="sy1">></span> sqrt<span class="br0">(</span><span class="kw4">const</span> valarray<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> va<span class="br0">)</span>
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<span class="br0">{</span>
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valarray<span class="sy1"><</span>T<span class="sy1">></span> other <span class="sy1">=</span> va<span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span>T <span class="sy3">&</span>i <span class="sy4">:</span> other<span class="br0">)</span> <span class="br0">{</span>
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i <span class="sy1">=</span> sqrt<span class="br0">(</span>i<span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> other<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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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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<p>寻找多个二次方程的实根。</p>
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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 <cstddef></span>
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<span class="co2">#include <valarray></span>
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<span class="co2">#include <iostream></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">double</span><span class="sy1">></span> a<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">8</span><span class="br0">)</span><span class="sy4">;</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">double</span><span class="sy1">></span> b<span class="br0">{</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="br0">}</span><span class="sy4">;</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">double</span><span class="sy1">></span> c <span class="sy1">=</span> <span class="sy2">-</span>b<span class="sy4">;</span>
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<span class="co1">// LWG3074 前字面量亦必须拥有 T 类型（此情况下为 double ）</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">double</span><span class="sy1">></span> d <span class="sy1">=</span> <a href="cpp-numeric-math-sqrt.html"><span class="kw1329">std::<span class="me2">sqrt</span></span></a><span class="br0">(</span>b <span class="sy2">*</span> b <span class="sy2">-</span> <span class="nu16">4.0</span> <span class="sy2">*</span> a <span class="sy2">*</span> c<span class="br0">)</span><span class="sy4">;</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">double</span><span class="sy1">></span> x1 <span class="sy1">=</span> <span class="br0">(</span><span class="sy2">-</span>b <span class="sy2">-</span> d<span class="br0">)</span> <span class="sy2">/</span> <span class="br0">(</span><span class="nu16">2.0</span> <span class="sy2">*</span> a<span class="br0">)</span><span class="sy4">;</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">double</span><span class="sy1">></span> x2 <span class="sy1">=</span> <span class="br0">(</span><span class="sy2">-</span>b <span class="sy2">+</span> d<span class="br0">)</span> <span class="sy2">/</span> <span class="br0">(</span><span class="nu16">2.0</span> <span class="sy2">*</span> a<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">"quadratic equation root 1, root 2"</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</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> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> a.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> <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> a<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">"x<span class="es3">\u00B2</span> + "</span> <span class="sy1"><<</span> b<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">"x + "</span> <span class="sy1"><<</span> c<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">" = 0 "</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> <a href="cpp-io-manip-fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span> x1<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1"><<</span> <span class="st0">", "</span> <span class="sy1"><<</span> x2<span class="br0">[</span>i<span class="br0">]</span> <span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1787">std::<span class="me2">defaultfloat</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>
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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">quadratic equation root 1, root 2
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1x² + 1x + -1 = 0 -1.618034, 0.618034
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1x² + 2x + -2 = 0 -2.732051, 0.732051
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1x² + 3x + -3 = 0 -3.791288, 0.791288
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1x² + 4x + -4 = 0 -4.828427, 0.828427
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1x² + 5x + -5 = 0 -5.854102, 0.854102
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1x² + 6x + -6 = 0 -6.872983, 0.872983
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1x² + 7x + -7 = 0 -7.887482, 0.887482
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1x² + 8x + -8 = 0 -8.898979, 0.898979</pre></div>
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</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-valarray-pow.html"><span class="t-lines"><span>pow<span class="t-dsc-small">(std::valarray)</span></span></span></a></div>
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</div>
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</td>
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<td>应用函数 <span class="t-lc"><a href="cpp-numeric-math-pow.html">std::pow</a></span> 到二个 valarray 或一个 valarray 与一个值<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-sqrt.html"><span class="t-lines"><span>sqrt</span><span>sqrtf</span><span>sqrtl</span></span></a></div>
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<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>计算平方根（ <span class="mjax" style="display:none">\(\small{\sqrt{x} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;"><span class="t-mrad"><span>√</span><span>x</span></span></span> ）<br />
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<span class="t-mark">(函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-complex-sqrt.html"><span class="t-lines"><span>sqrt<span class="t-dsc-small">(std::complex)</span></span></span></a></div>
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</div>
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</td>
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<td>右半平面范围中的复平方根<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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