222 lines
17 KiB
HTML
222 lines
17 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::frexp, std::frexpf, std::frexpl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_math_frexp 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>frexp, <span style="font-size:0.7em; line-height:130%">std::</span>frexpf, <span style="font-size:0.7em; line-height:130%">std::</span>frexpl</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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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> frexp <span class="br0">(</span> <span class="kw4">float</span> arg, <span class="kw4">int</span><span class="sy2">*</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> frexpf<span class="br0">(</span> <span class="kw4">float</span> arg, <span class="kw4">int</span><span class="sy2">*</span> exp <span class="br0">)</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"><span class="kw4">double</span> frexp <span class="br0">(</span> <span class="kw4">double</span> arg, <span class="kw4">int</span><span class="sy2">*</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td rowspan="142">(3)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> frexp <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg, <span class="kw4">int</span><span class="sy2">*</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> frexpl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg, <span class="kw4">int</span><span class="sy2">*</span> exp <span class="br0">)</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"><span class="kw4">double</span> frexp <span class="br0">(</span> IntegralType arg, <span class="kw4">int</span><span class="sy2">*</span> exp <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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<div class="t-li1"><span class="t-li">1-3)</span> 分解给定的浮点值 <code>x</code> 为正规化小数和二的整数幂。</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 <span class="t-v">(2)</span> （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>浮点值</td>
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</tr>
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<tr class="t-par">
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<td>exp</td>
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<td>-</td>
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<td>指向要存储指数到的整数的指针</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 <code>arg</code> 为零，则返回零并存储零于 <code>*exp</code> 。</p>
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<p>否则（若 <code>arg</code> 非零），若不出现错误，则返回范围 <code>(-1;-0.5], [0.5; 1)</code> 中的值 <code>x</code> ，并存储整数值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>exp</span></span> ，满足 <span class="texhtml" style="white-space: nowrap;">x×2<span class="t-su">(*exp)<br /></span>=arg</span> 。</p>
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<p>若存储于 <code>*exp</code> 的值在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> 范围外，则行为未指定。</p>
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<p>若 <code>arg</code> 不是浮点数，则行为未指定。</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>此函数不受制于任何指定于 <span class="t-lc"><a href="cpp-numeric-math-math_errhandling.html">math_errhandling</a></span> 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 <code>arg</code> 为 ±0 ，则返回不修改的参数，并存储 <code>0</code> 于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>exp</span></span> 。</li>
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<li>若 <code>arg</code> 为 ±∞ ，则返回它，并存储未指定值于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>exp</span></span> 。</li>
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<li>若 <code>arg</code> 为 NaN ，则返回 NaN ，并存储未指定值于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>exp</span></span> 。</li>
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<li>不引发浮点异常。</li>
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<li>若 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 为 2 （或 2 的幂），则返回值准确，忽略<a href="cpp-numeric-fenv-FE_round.html">当前舍入模式</a>。</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>二进制系统（其中 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 为 <code>2</code> ）上， <code>frexp</code> 可实现为</p>
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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="br0">{</span>
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<span class="sy2">*</span>exp <span class="sy1">=</span> <span class="br0">(</span>value <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span> <span class="sy4">?</span> <span class="nu0">0</span> <span class="sy4">:</span> <span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="br0">(</span><span class="nu0">1</span> <span class="sy2">+</span> <a href="cpp-numeric-math-logb.html"><span class="kw1328">std::<span class="me2">logb</span></span></a><span class="br0">(</span>value<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="cpp-numeric-math-scalbn.html"><span class="kw1362">std::<span class="me2">scalbn</span></span></a><span class="br0">(</span>value, <span class="sy2">-</span><span class="br0">(</span><span class="sy2">*</span>exp<span class="br0">)</span><span class="br0">)</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>函数 <code>std::frexp</code> 与其对偶 <span class="t-lc"><a href="cpp-numeric-math-ldexp.html">std::ldexp</a></span> 能一起用于操纵浮点数的表示，而无需直接的位操作。</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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<p>比较不同的浮点分解函数</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 <iostream></span>
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<span class="co2">#include <cmath></span>
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<span class="co2">#include <limits></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="kw4">double</span> f <span class="sy1">=</span> <span class="nu16">123.45</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">"Given the number "</span> <span class="sy1"><<</span> f <span class="sy1"><<</span> <span class="st0">" or "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1785">std::<span class="me2">hexfloat</span></span></a>
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<span class="sy1"><<</span> f <span class="sy1"><<</span> <a href="cpp-io-manip-fixed.html"><span class="kw1786">std::<span class="me2">defaultfloat</span></span></a> <span class="sy1"><<</span> <span class="st0">" in hex,<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw4">double</span> f3<span class="sy4">;</span>
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<span class="kw4">double</span> f2 <span class="sy1">=</span> <a href="cpp-numeric-math-modf.html"><span class="kw1361">std::<span class="me2">modf</span></span></a><span class="br0">(</span>f, <span class="sy3">&</span>f3<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">"modf() makes "</span> <span class="sy1"><<</span> f3 <span class="sy1"><<</span> <span class="st0">" + "</span> <span class="sy1"><<</span> f2 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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f2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">frexp</span><span class="br0">(</span>f, <span class="sy3">&</span>i<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">"frexp() makes "</span> <span class="sy1"><<</span> f2 <span class="sy1"><<</span> <span class="st0">" * 2^"</span> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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i <span class="sy1">=</span> <a href="cpp-numeric-math-ilogb.html"><span class="kw1327">std::<span class="me2">ilogb</span></span></a><span class="br0">(</span>f<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">"logb()/ilogb() make "</span> <span class="sy1"><<</span> f<span class="sy2">/</span><a href="cpp-numeric-math-scalbn.html"><span class="kw1362">std::<span class="me2">scalbn</span></span></a><span class="br0">(</span><span class="nu16">1.0</span>, i<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" * "</span>
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<span class="sy1"><<</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">radix</span>
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<span class="sy1"><<</span> <span class="st0">"^"</span> <span class="sy1"><<</span> <a href="cpp-numeric-math-ilogb.html"><span class="kw1327">std::<span class="me2">ilogb</span></span></a><span class="br0">(</span>f<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>可能的输出：</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">Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
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modf() makes 123 + 0.45
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frexp() makes 0.964453 * 2^7
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logb()/ilogb() make 1.92891 * 2^6</pre></div>
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</div>
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</div>
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<p><br /></p>
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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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-ldexp.html"><span class="t-lines"><span>ldexp</span><span>ldexpf</span><span>ldexpl</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>将数乘以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 的幂次<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-logb.html"><span class="t-lines"><span>logb</span><span>logbf</span><span>logbl</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>提取数的指数<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-ilogb.html"><span class="t-lines"><span>ilogb</span><span>ilogbf</span><span>ilogbl</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>提取数的指数<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-modf.html"><span class="t-lines"><span>modf</span><span>modff</span><span>modfl</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>分解数为整数和小数部分<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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</div>
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</div>
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</div>
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</body>
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