179 lines
17 KiB
HTML
179 lines
17 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::logb, std::logbf, std::logbl</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_math_logb skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>logb, <span style="font-size:0.7em; line-height:130%">std::</span>logbf, <span style="font-size:0.7em; line-height:130%">std::</span>logbl</h1>
|
|
<div id="bodyContent">
|
|
<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>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><a href="cpp-header-cmath.html"><cmath></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> logb <span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span><br />
|
|
<span class="kw4">float</span> logbf<span class="br0">(</span> <span class="kw4">float</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(1)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> logb <span class="br0">(</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(2)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> logb <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> arg <span class="br0">)</span><span class="sy4">;</span><br />
|
|
<span class="kw4">long</span> <span class="kw4">double</span> logbl<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>
|
|
</td>
|
|
<td>(3)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> logb <span class="br0">(</span> IntegralType arg <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(4)</td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<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，并将它作为浮点值返回。</div>
|
|
<div class="t-li1"><span class="t-li">4)</span> 接收任何<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>
|
|
<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 />
|
|
r</span>|arg|</span> 的有符号整数部分（此函数作为浮点值返回），其中 <code>r</code> 是 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 。若 <code>arg</code> 为非正规，则当做它如同已正规化。</p>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
|
<table class="t-par-begin">
|
|
<tr class="t-par">
|
|
<td>arg</td>
|
|
<td>-</td>
|
|
<td>浮点值</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p>若不出现错误，则返回作为有符号浮点值的 <code>arg</code> 的无偏指数。</p>
|
|
<p>若出现定义域错误，则返回实现定义值。</p>
|
|
<p>若出现极点错误，则返回 <code>-HUGE_VAL</code> 、 <code>-HUGE_VALF</code> 或 <code>-HUGE_VALL</code> 。</p>
|
|
<h3><span class="mw-headline" id=".E9.94.99.E8.AF.AF.E5.A4.84.E7.90.86">错误处理</span></h3>
|
|
<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>
|
|
<p>若 <code>arg</code> 为零则可能出现定义域或值域错误。</p>
|
|
<p>若实现支持 IEEE 浮点算术（ IEC 60559 ），则</p>
|
|
<ul>
|
|
<li>若 <code>arg</code> 为 ±0 ，则返回 -∞ 并引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_DIVBYZERO</a></span> 。</li>
|
|
<li>若 <code>arg</code> 为 ±∞ ，则返回 +∞ 。</li>
|
|
<li>若 <code>arg</code> 为 NaN ，则返回 NaN 。</li>
|
|
<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> ）且忽略<a href="cpp-numeric-fenv-FE_round.html">当前舍入模式</a>。</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
|
|
<p><a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/logb.html">POSIX 要求</a>若 <code>arg</code> 为 ±0 则出现极点错误。</p>
|
|
<p><code>std::logb</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>logb</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">frexp</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>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<p>比较不同的浮点分解函数</p>
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <cmath></span>
|
|
<span class="co2">#include <limits></span>
|
|
<span class="co2">#include <cfenv></span>
|
|
<span class="co2">#pragma STDC FENV_ACCESS ON</span>
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">double</span> f <span class="sy1">=</span> <span class="nu16">123.45</span><span class="sy4">;</span>
|
|
<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>
|
|
<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>
|
|
|
|
<span class="kw4">double</span> f3<span class="sy4">;</span>
|
|
<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>
|
|
<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>
|
|
|
|
<span class="kw4">int</span> i<span class="sy4">;</span>
|
|
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>
|
|
<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>
|
|
|
|
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>
|
|
<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>
|
|
<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>
|
|
<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>
|
|
|
|
<span class="co1">// 错误处理</span>
|
|
<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>
|
|
<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(0) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">logb</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>
|
|
<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="kw1461">FE_DIVBYZERO</span></a><span class="br0">)</span><span class="br0">)</span>
|
|
<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_DIVBYZERO raised<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">Given the number 123.45 or 0x1.edccccccccccdp+6 in hex,
|
|
modf() makes 123 + 0.45
|
|
frexp() makes 0.964453 * 2^7
|
|
logb()/ilogb() make 1.92891 * 2^6
|
|
logb(0) = -Inf
|
|
FE_DIVBYZERO raised</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-numeric-math-frexp.html"><span class="t-lines"><span>frexp</span><span>frexpf</span><span>frexpl</span></span></a></div>
|
|
<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>
|
|
</div>
|
|
</td>
|
|
<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 />
|
|
<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-ilogb.html"><span class="t-lines"><span>ilogb</span><span>ilogbf</span><span>ilogbl</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>
|
|
</div>
|
|
</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>
|
|
<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>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|