166 lines
14 KiB
HTML
166 lines
14 KiB
HTML
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<title>std::cbrt, std::cbrtf, std::cbrtl</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>cbrt, <span style="font-size:0.7em; line-height:130%">std::</span>cbrtf, <span style="font-size:0.7em; line-height:130%">std::</span>cbrtl</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-math.html">math</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-cmath.html"><cmath></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> cbrt <span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">float</span> cbrtf<span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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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"><span class="kw4">double</span> cbrt <span class="br0">(</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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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"><span class="kw4">long</span> <span class="kw4">double</span> cbrt <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">long</span> <span class="kw4">double</span> cbrtl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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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"><span class="kw4">double</span> cbrt <span class="br0">(</span> IntegralType arg <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl-sep">
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<div class="t-li1"><span class="t-li">1-3)</span> 计算 <code>arg</code> 的立方根。</div>
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<div class="t-li1"><span class="t-li">4)</span> 接受任何<a href="cpp-types-is_integral.html">整数类型</a>参数的重载集或函数模板。等价于 2) （将参数转型为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span></span></span> ）。</div>
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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>arg</td>
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<td>-</td>
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<td>浮点或<a href="cpp-types-is_integral.html">整数类型</a>值</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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<p>若不出现错误，则返回 <code>arg</code> 的立方根（ <span class="mjax" style="display:none">\(\small{\sqrt[3]{arg} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;"><span class="t-mrad"><span>3</span><span>√</span><span>arg</span></span></span> ）。</p>
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<p>若出现下溢所致的错误，则返回（舍入后的）正确结果。</p>
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<h3><span class="mw-headline" id=".E9.94.99.E8.AF.AF.E5.A4.84.E7.90.86">错误处理</span></h3>
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<p>报告 <span class="t-lc"><a href="cpp-numeric-math-math_errhandling.html">math_errhandling</a></span> 中指定的错误。</p>
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<p>若实现支持 IEEE 浮点算术（ IEC 60559 ），则</p>
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<ul>
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<li>若参数为 ±0 或 ±∞ ，则返回不更改的参数</li>
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<li>若参数为 NaN ，则返回 NaN 。</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span>arg<span class="br0">)</span></span></span> 不等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-math-pow.html"><span class="kw1331">std::<span class="me2">pow</span></span></a><span class="br0">(</span>arg, <span class="nu16">1.0</span><span class="sy2">/</span><span class="nu0">3</span><span class="br0">)</span></span></span> ，因为有理数 <span class="mjax" style="display:none">\(\small{\frac1{3} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;"><span class="t-mfrac"></span></span>
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<table>
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<td>1</td>
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<td>3</td>
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通常不等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">1.0</span><span class="sy2">/</span><span class="nu0">3</span></span></span> 并且 <span class="t-lc"><a href="cpp-numeric-math-pow.html">std::pow</a></span> 不能求负底数的小数次幂。另外 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span>arg<span class="br0">)</span></span></span> 常给出比 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-numeric-math-pow.html"><span class="kw1331">std::<span class="me2">pow</span></span></a><span class="br0">(</span>arg, <span class="nu16">1.0</span><span class="sy2">/</span><span class="nu0">3</span><span class="br0">)</span></span></span> 更精确的结果（见示例）。
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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 <cmath></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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<span class="co1">// 正常使用</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"cbrt(729) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span><span class="nu0">729</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"cbrt(-0.125) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.125</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// 特殊值</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"cbrt(-0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.0</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"cbrt(+inf) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span><a href="cpp-numeric-math-INFINITY.html"><span class="kw1376">INFINITY</span></a><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// 精度</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a>.<span class="me1">precision</span><span class="br0">(</span><a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><span class="kw4">double</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">max_digits10</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"cbrt(343) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">cbrt</span><span class="br0">(</span><span class="nu0">343</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"pow(343,1.0/3) = "</span> <span class="sy1"><<</span> <a href="cpp-numeric-math-pow.html"><span class="kw1331">std::<span class="me2">pow</span></span></a><span class="br0">(</span><span class="nu0">343</span>, <span class="nu16">1.0</span><span class="sy2">/</span><span class="nu0">3</span><span class="br0">)</span> <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">cbrt(729) = 9
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cbrt(-0.125) = -0.5
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cbrt(-0) = -0
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cbrt(+inf) = inf
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cbrt(343) = 7
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pow(343,1.0/3) = 6.9999999999999991</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-pow.html"><span class="t-lines"><span>pow</span><span>powf</span><span>powl</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>求某数的给定次幂（ <span class="mjax" style="display:none">\(\small{x^y}\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">x<sup>y</sup></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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<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 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-hypot.html"><span class="t-lines"><span>hypot</span><span>hypotf</span><span>hypotl</span></span></a></div>
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<div><span class="t-lines"><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><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>计算两个给定数的平方和的平方根（ <span class="mjax" style="display:none">\(\scriptsize{\sqrt{x^2+y^2} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;"><span class="t-mrad"><span>√</span><span>x<span class="t-su">2<br /></span>+y<span class="t-su">2<br /></span></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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</table>
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</div>
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<div class="visualClear"></div>
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