201 lines
22 KiB
HTML
201 lines
22 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>std::ilogb, std::ilogbf, std::ilogbl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_math_ilogb 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>ilogb, <span style="font-size:0.7em; line-height:130%">std::</span>ilogbf, <span style="font-size:0.7em; line-height:130%">std::</span>ilogbl</h1>
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<div id="bodyContent">
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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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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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>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> ilogb <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">int</span> ilogbf<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>
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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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</tr>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> ilogb <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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</tr>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> ilogb <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">int</span> ilogbl<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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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> ilogb <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 t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="co2">#define FP_ILOGB0 /*implementation-defined*/</span></span></div>
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<td>(5)</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="co2">#define FP_ILOGBNAN /*implementation-defined*/</span></span></div>
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<td>(6)</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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<td></td>
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<div class="t-li1"><span class="t-li">1-3)</span> 从浮点参数 <code>arg</code> 提取独立于基底的无偏指数，并将它作为有符号整数值返回。</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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<div class="t-li1"><span class="t-li">5)</span> 展开成整数常量表达式，值为 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><a href="cpp-types-climits.html"><span class="kw398">INT_MAX</span></a></span></span> 。</div>
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<div class="t-li1"><span class="t-li">6)</span> 展开成整数常量表达式，值为 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span><a href="cpp-types-climits.html"><span class="kw398">INT_MAX</span></a></span></span> 。</div>
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<p>正式而言，无偏指数是非零 arg 的 <span class="texhtml" style="white-space: nowrap;">log<span class="t-su t-su-b"><br />
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r</span>|arg|</span> 的整数部分，作为有符号整数值，其中 <code>r</code> 是 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">radix</span></span></span> ，而 <code>T</code> 是 <code>arg</code> 的浮点类型。</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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<tr class="t-par">
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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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</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>若不出现错误，则返回作为 int 值的 <code>arg</code> 的无偏指数。</p>
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<p>若 <code>arg</code> 为零，则返回 <span class="t-lc">FP_ILOGB0</span> 。</p>
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<p>若 <code>arg</code> 为无穷大，则返回 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 。</p>
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<p>若 <code>arg</code> 为 NaN ，则返回 <span class="t-lc">FP_ILOGBNAN</span> 。</p>
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<p>若正确结果大于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 或小于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> ，则返回值未指定，且可能出现定义域或值域错误。</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 <code>arg</code> 为零、无穷大或 NaN ，则可能出现定义域错误或值域错误。</p>
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<p>若正确结果大于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 或小于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> ，则可能出现定义域错误或值域错误。</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>若正确结果大于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 或小于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> ，则引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_INVALID</a></span> 。</li>
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<li>若 <code>arg</code> 为 ±0 、 ±∞ 或 NaN ，则引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_INVALID</a></span> 。</li>
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<li>所有其他情况下，结果是准确的（决不引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_INEXACT</a></span> ）并且忽略<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>若 <code>arg</code> 不是零、无穷大或 NaN ，则返回的值准确等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><a href="cpp-numeric-math-logb.html"><span class="kw1327">std::<span class="me2">logb</span></span></a><span class="br0">(</span>arg<span class="br0">)</span></span></span> 。</p>
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<p><a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/ilogb.html">POSIX 要求</a>若 <code>arg</code> 为零、无穷大、 NaN 或若正确结果在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> 的范围外则出现定义域错误。</p>
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<p>POSIX 亦要求在符合 XSI 的系统上，正确结果大于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 时返回值为 <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> ，而正确结果小于 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> 时返回值为 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> 。</p>
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<p>在所有已知平台上正确结果都能表示成 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span></span></span> 。对于要出现溢出的情况， <span class="t-lc"><a href="cpp-types-climits.html">INT_MAX</a></span> 必须小于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-climits.html"><span class="kw437">LDBL_MAX_EXP</span></a><span class="sy2">*</span>log2<span class="br0">(</span><a href="cpp-types-climits.html"><span class="kw406">FLT_RADIX</span></a><span class="br0">)</span></span></span> 或 <span class="t-lc"><a href="cpp-types-climits.html">INT_MIN</a></span> 必须大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">LDBL_MIN_EXP<span class="sy2">-</span><a href="cpp-types-climits.html"><span class="kw428">LDBL_MANT_DIG</span></a><span class="br0">)</span><span class="sy2">*</span>log2<span class="br0">(</span><a href="cpp-types-climits.html"><span class="kw406">FLT_RADIX</span></a><span class="br0">)</span></span></span> 。</p>
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<p><code>std::ilogb</code> 所返回的指数值始终比 <span class="t-lc"><a href="cpp-numeric-math-frexp.html">std::frexp</a></span> 所返回的小 1 ，因为不同的正规化要求：对于 <code>ilogb</code> 返回的指数 <code>e</code> ， <span class="texhtml" style="white-space: nowrap;">|arg*r<span class="t-su">-e<br /></span>|</span> 在 1 与 <code>r</code> 之间（典型地在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">2</span></span></span> 之间），但 <span class="t-lc"><a href="cpp-numeric-math-frexp.html">std::frexp</a></span> 返回的指数 <code>e</code> ， <span class="texhtml" style="white-space: nowrap;">|arg*2<span class="t-su">-e<br /></span>|</span> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">0.5</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu0">1</span></span></span> 之间。</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="co2">#include <cfenv></span>
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<span class="co2">#pragma STDC FENV_ACCESS ON</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> <a href="cpp-numeric-math-frexp.html"><span class="kw1359">std::<span class="me2">frexp</span></span></a><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> std<span class="sy4">::</span><span class="me2">ilogb</span><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> std<span class="sy4">::</span><span class="me2">ilogb</span><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="co1">// 错误处理</span>
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<a href="cpp-numeric-fenv-feclearexcept.html"><span class="kw1449">std::<span class="me2">feclearexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1460">FE_ALL_EXCEPT</span></a><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">"ilogb(0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ilogb</span><span class="br0">(</span><span class="nu0">0</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="kw1">if</span> <span class="br0">(</span><a href="cpp-numeric-fenv-fetestexcept.html"><span class="kw1450">std::<span class="me2">fetestexcept</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_exceptions.html"><span class="kw1463">FE_INVALID</span></a><span class="br0">)</span><span class="br0">)</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">" FE_INVALID raised<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
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ilogb(0) = -2147483648
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FE_INVALID raised</pre></div>
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</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-math-frexp.html"><span class="t-lines"><span>frexp</span><span>frexpf</span><span>frexpl</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>将数分解为有效数字和 <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>
|
|
<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>
|
|
<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>
|
|
<td>提取数的指数<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-numeric-math-scalbn.html"><span class="t-lines"><span>scalbn</span><span>scalbnf</span><span>scalbnl</span><span>scalbln</span><span>scalblnf</span><span>scalblnl</span></span></a></div>
|
|
<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><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>
|
|
</div>
|
|
</td>
|
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<td>将数乘以 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 的幂次<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
</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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</html>
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