202 lines
16 KiB
HTML
202 lines
16 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::ldexp, std::ldexpf, std::ldexpl</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_ldexp 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>ldexp, <span style="font-size:0.7em; line-height:130%">std::</span>ldexpf, <span style="font-size:0.7em; line-height:130%">std::</span>ldexpl</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>
|
|
</tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(1)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl">
|
|
<td class="t-dcl-nopad">
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> ldexp <span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td class="t-dcl-nopad"></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> ldexpf<span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody>
|
|
<tr class="t-dcl">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> ldexp <span class="br0">(</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(2)</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody class="t-dcl-rev t-dcl-rev-num">
|
|
<tr class="t-dcl-rev-aux">
|
|
<td></td>
|
|
<td rowspan="142">(3)</td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl">
|
|
<td class="t-dcl-nopad">
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> ldexp <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td class="t-dcl-nopad"></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> ldexpl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">int</span> exp <span class="br0">)</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td class="t-dcl-nopad"></td>
|
|
<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
|
|
</tr>
|
|
</tbody>
|
|
<tbody>
|
|
<tr class="t-dcl t-since-cxx11">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> ldexp <span class="br0">(</span> IntegralType x, <span class="kw4">int</span> exp <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> 将浮点值 <code>arg</code> 乘以 2 的 <code>exp</code> 次幂。</div>
|
|
<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>
|
|
<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>
|
|
<tr class="t-par">
|
|
<td>exp</td>
|
|
<td>-</td>
|
|
<td>整数值</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> 乘 2 的 <code>exp</code> 次幂（ <span class="texhtml" style="white-space: nowrap;">arg×2<span class="t-su">exp<br /></span></span> ）。</p>
|
|
<p>若出现上溢所致的值域错误，则返回 <code>±HUGE_VAL</code> 、 <code>±HUGE_VALF</code> 或 <code>±HUGE_VALL</code> 。</p>
|
|
<p>若出现下溢所致的值域错误，则返回（舍入后的）正确结果。</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>若实现支持 IEEE 浮点算术（ IEC 60559 ），则</p>
|
|
<ul>
|
|
<li>决不引发 <span class="t-lc"><a href="cpp-numeric-fenv-FE_exceptions.html">FE_INEXACT</a></span> ，除非出现值域错误（结果准确）</li>
|
|
<li>忽略<a href="cpp-numeric-fenv-FE_round.html">当前舍入模式</a>，除非出现值域错误</li>
|
|
<li>若 <code>arg</code> 为 ±0 ，则返回不修改的参数</li>
|
|
<li>若 <code>arg</code> 为 ±∞ ，则返回不修改的参数</li>
|
|
<li>若 <code>exp</code> 为 0 ，则返回不修改的 <code>arg</code></li>
|
|
<li>若 <code>arg</code> 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>二进制系统上（其中 <span class="t-lc"><a href="cpp-types-climits.html">FLT_RADIX</a></span> 为 <code>2</code> ）， <code>std::ldexp</code> 等价于 <span class="t-lc"><a href="cpp-numeric-math-scalbn.html">std::scalbn</a></span> 。</p>
|
|
<p>函数 <code>std::ldexp</code> （“加载指数”）与其对偶 <span class="t-lc"><a href="cpp-numeric-math-frexp.html">std::frexp</a></span> 能一同用于操纵浮点数的表示，而无需直接的位操作。</p>
|
|
<p>多数实现上， <code>std::ldexp</code> 效率低于用通常算术运算符乘或除以二的幂。</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="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <cmath></span>
|
|
<span class="co2">#include <cerrno></span>
|
|
<span class="co2">#include <cstring></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>
|
|
<a href="cpp-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"ldexp(7, -4) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><span class="nu0">7</span>, <span class="sy2">-</span><span class="nu0">4</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
|
|
<span class="sy1"><<</span> <span class="st0">"ldexp(1, -1074) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><span class="nu0">1</span>, <span class="sy2">-</span><span class="nu0">1074</span><span class="br0">)</span>
|
|
<span class="sy1"><<</span> <span class="st0">" (minimum positive subnormal double)<span class="es1">\n</span>"</span>
|
|
<span class="sy1"><<</span> <span class="st0">"ldexp(nextafter(1,0), 1024) = "</span>
|
|
<span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><a href="cpp-numeric-math-nextafter.html"><span class="kw1364">std::<span class="me2">nextafter</span></span></a><span class="br0">(</span><span class="nu0">1</span>,<span class="nu0">0</span><span class="br0">)</span>, <span class="nu0">1024</span><span class="br0">)</span>
|
|
<span class="sy1"><<</span> <span class="st0">" (largest finite double)<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="co1">// 特殊值</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">"ldexp(-0, 10) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.0</span>, <span class="nu0">10</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
|
|
<span class="sy1"><<</span> <span class="st0">"ldexp(-Inf, -1) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><span class="sy2">-</span><a href="cpp-numeric-math-INFINITY.html"><span class="kw1376">INFINITY</span></a>, <span class="sy2">-</span><span class="nu0">1</span><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>
|
|
<span class="kw2">errno</span> <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</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">"ldexp(1, 1024) = "</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">ldexp</span><span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">1024</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><span class="kw2">errno</span> <span class="sy1">==</span> <a href="cpp-error-errno_macros.html"><span class="kw862">ERANGE</span></a><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">" errno == ERANGE: "</span> <span class="sy1"><<</span> <a href="cpp-string-byte-strerror.html"><span class="kw1161">std::<span class="me2">strerror</span></span></a><span class="br0">(</span><span class="kw2">errno</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="kw1464">FE_OVERFLOW</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_OVERFLOW raised<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">ldexp(7, -4) = 0.4375
|
|
ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
|
|
ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
|
|
ldexp(-0, 10) = -0
|
|
ldexp(-Inf, -1) = -inf
|
|
ldexp(1, 1024) = inf
|
|
errno == ERANGE: Numerical result out of range
|
|
FE_OVERFLOW 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-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>
|