240 lines
15 KiB
HTML
240 lines
15 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>log2, log2f, log2l</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-c_numeric_math_log2 skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading">log2, log2f, log2l</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-numeric.html">numeric</a>‎ | <a href="c-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><math.h></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-c99">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c"><span class="kw4">float</span> log2f<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-c99">(C99 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-c99">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c"><span class="kw4">double</span> log2<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-c99">(C99 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-c99">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c"><span class="kw4">long</span> <span class="kw4">double</span> log2l<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-c99">(C99 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><tgmath.h></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-c99">
|
|
<td>
|
|
<div><span class="mw-geshi c source-c"><span class="co2">#define log2( arg )</span></span></div>
|
|
</td>
|
|
<td>(4)</td>
|
|
<td><span class="t-mark-rev t-since-c99">(C99 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> 计算 <code>arg</code> 的底 <i>2</i> 对数。</div>
|
|
<div class="t-li1"><span class="t-li">4)</span> 泛型宏：若 <code>arg</code> 拥有 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">long</span> <span class="kw4">double</span></span></span> 类型，则调用 <code>log2l</code> 。否则，若 <code>arg</code> 拥有整数类型或 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">double</span></span></span> 类型，则调用 <code>log2</code> 。否则调用 <code>log2f</code> 。</div>
|
|
<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 <code>arg</code> 的底 <i>2</i> 对数（ <span class="texhtml" style="white-space: nowrap;">log<span class="t-su t-su-b"><br />
|
|
2</span>(arg)</span> 或 <span class="texhtml" style="white-space: nowrap;">lb(arg)</span> ）。</p>
|
|
<p>若出现定义域错误，则返回实现定义值（支持的平台上为 NaN ）。</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 <a href="c-numeric-math-math_errhandling.html">math_errhandling</a> 中指定的错误。</p>
|
|
<p>若 <code>arg</code> 小于零则出现定义域错误。</p>
|
|
<p>若 <code>arg</code> 为零则可能出现极点错误。</p>
|
|
<p>若实现支持 IEEE 浮点算术（ IEC 60559 ），则</p>
|
|
<ul>
|
|
<li>若参数为 ±0 ，则返回 -∞ 并引发 <span class="t-lc"><a href="c-numeric-fenv-FE_exceptions.html">FE_DIVBYZERO</a></span> 。</li>
|
|
<li>若参数为 1 ，则返回 +0 。</li>
|
|
<li>若参数为负数，则返回 NaN 并引发 <span class="t-lc"><a href="c-numeric-fenv-FE_exceptions.html">FE_INVALID</a></span> 。</li>
|
|
<li>若参数为 +∞ ，则返回 +∞ 。</li>
|
|
<li>若参数为 NaN ，则返回 NaN 。</li>
|
|
</ul>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
|
|
<p>对于整数 <code>arg</code> ，二进制对数能转译成输入中最高位 1 的零底下标。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="c source-c">
|
|
<pre class="de1"><span class="co2">#include <stdio.h></span>
|
|
<span class="co2">#include <math.h></span>
|
|
<span class="co2">#include <float.h></span>
|
|
<span class="co2">#include <errno.h></span>
|
|
<span class="co2">#include <fenv.h></span>
|
|
<span class="co2">#pragma STDC FENV_ACCESS ON</span>
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(65536) = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu0">65536</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(0.125) = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu16">0.125</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(0x020f) = %f (highest set bit is in position 9)<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu12">0x020f</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"base-5 logarithm of 125 = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu0">125</span><span class="br0">)</span><span class="sy2">/</span>log2<span class="br0">(</span><span class="nu0">5</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="co1">// 特殊值</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(1) = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(+Inf) = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span>INFINITY<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="co1">// 错误处理</span>
|
|
<span class="kw2">errno</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <a href="c-numeric-fenv-feclearexcept.html"><span class="kw718">feclearexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw729">FE_ALL_EXCEPT</span></a><span class="br0">)</span><span class="sy4">;</span>
|
|
<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"log2(0) = %f<span class="es1">\n</span>"</span>, log2<span class="br0">(</span><span class="nu0">0</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">if</span><span class="br0">(</span><span class="kw2">errno</span> <span class="sy1">==</span> <a href="c-error-errno_macros.html"><span class="kw478">ERANGE</span></a><span class="br0">)</span> <a href="c-io-perror.html"><span class="kw879">perror</span></a><span class="br0">(</span><span class="st0">" errno == ERANGE"</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw719">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw730">FE_DIVBYZERO</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">" FE_DIVBYZERO raised"</span><span class="br0">)</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">log2(65536) = 16.000000
|
|
log2(0.125) = -3.000000
|
|
log2(0x020f) = 9.041659 (highest set bit is in position 9)
|
|
base-5 logarithm of 125 = 3.000000
|
|
log2(1) = 0.000000
|
|
log2(+Inf) = inf
|
|
log2(0) = -inf
|
|
errno == ERANGE: Numerical result out of range
|
|
FE_DIVBYZERO raised</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
|
|
<div class="t-ref-std-11">
|
|
<ul>
|
|
<li>C11 标准（ISO/IEC 9899:2011）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.12.6.10 The log2 functions (p: 246)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.25 Type-generic math <tgmath.h> (p: 373-375)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>F.10.3.10 The log2 functions (p: 522)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<div class="t-ref-std-c99">
|
|
<ul>
|
|
<li>C99 标准（ISO/IEC 9899:1999）：</li>
|
|
</ul>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.12.6.10 The log2 functions (p: 226)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>7.22 Type-generic math <tgmath.h> (p: 335-337)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
<dl>
|
|
<dd>
|
|
<ul>
|
|
<li>F.9.3.10 The log2 functions (p: 459)</li>
|
|
</ul>
|
|
</dd>
|
|
</dl>
|
|
</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="c-numeric-math-log.html"><span class="t-lines"><span>log</span><span>logf</span><span>logl</span></span></a></div>
|
|
<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>计算自然对数（底为 <i>e</i> ）（ <span class="mjax" style="display:none">\({\small \ln{x} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">ln(x)</span> ）<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-numeric-math-log10.html"><span class="t-lines"><span>log10</span><span>log10f</span><span>log10l</span></span></a></div>
|
|
<div><span class="t-lines"><span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>计算常用对数 （底为 <i>10</i> ）（ <span class="mjax" style="display:none">\({\small \log_{10}{x} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">log<sub>10</sub>(x)</span> ）<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-numeric-math-log1p.html"><span class="t-lines"><span>log1p</span><span>log1pf</span><span>log1pl</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>计算给定数加 1 的自然对数（底为 <i>e</i> ）（ <span class="mjax" style="display:none">\({\small \ln{(1+x)} }\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">ln(1+x)</span> ）<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="c-numeric-math-exp2.html"><span class="t-lines"><span>exp2</span><span>exp2f</span><span>exp2l</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>计算 <i>2</i> 的给定次幂（ <span class="mjax" style="display:none">\({\small 2^x}\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">2<sup>x</sup></span> ）<br />
|
|
<span class="t-mark">(函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|