237 lines
15 KiB
HTML
237 lines
15 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>ldexp, ldexpf, ldexpl</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_numeric_math_ldexp skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">ldexp, ldexpf, ldexpl</h1>
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<div id="bodyContent">
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<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>
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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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<td>
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<div>定义于头文件 <code><math.h></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi c source-c"><span class="kw4">float</span> ldexpf<span class="br0">(</span> <span class="kw4">float</span> arg, <span class="kw4">int</span> <a href="c-numeric-math-exp.html"><span class="kw657">exp</span></a> <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-c99">(C99 起)</span></td>
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<tr class="t-dcl">
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<td>
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<div><span class="mw-geshi c source-c"><span class="kw4">double</span> ldexp<span class="br0">(</span> <span class="kw4">double</span> arg, <span class="kw4">int</span> <a href="c-numeric-math-exp.html"><span class="kw657">exp</span></a> <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td class="t-dcl-nopad"></td>
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<td>
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<div><span class="mw-geshi c source-c"><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> arg, <span class="kw4">int</span> <a href="c-numeric-math-exp.html"><span class="kw657">exp</span></a> <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tr>
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<tr class="t-dsc-header">
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<td>
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<div>定义于头文件 <code><tgmath.h></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-c99">
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<div><span class="mw-geshi c source-c"><span class="co2">#define ldexp( arg, exp )</span></span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-c99">(C99 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tbody>
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<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>
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<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>ldexpl</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>ldexp</code> 。否则调用 <code>ldexpf</code> 。</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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<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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<tr class="t-par">
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<td>exp</td>
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<td>-</td>
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<td>整数值</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 <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>
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<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>
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<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>
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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 <a href="c-numeric-math-math_errhandling.html">math_errhandling</a> 中指定的错误。</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>决不引发 <span class="t-lc"><a href="c-numeric-fenv-FE_exceptions.html">FE_INEXACT</a></span> ，除非出现值域错误（结果准确）</li>
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<li>忽略<a href="c-numeric-fenv-FE_round.html">当前舍入模式</a>，除非出现值域错误</li>
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<li>若 <code>arg</code> 为 ±0 ，则返回不修改的参数</li>
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<li>若 <code>arg</code> 为 ±∞ ，则返回不修改的参数</li>
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<li>若 <code>exp</code> 为 0 ，则返回不修改的 <code>arg</code></li>
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<li>若 <code>arg</code> 为 NaN ，则返回 NaN</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>二进制系统上（其中 <span class="t-lc"><a href="c-types-limits.html">FLT_RADIX</a></span> 为 <code>2</code> ）， <code>ldexp</code> 等价于 <span class="t-lc"><a href="c-numeric-math-scalbn.html">scalbn</a></span> 。</p>
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<p>函数 <code>ldexp</code> （“加载指数”）与其对偶 <span class="t-lc"><a href="c-numeric-math-frexp.html">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>
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<p>多数实现上， <code>ldexp</code> 效率低于用通常算术运算符乘或除以二的幂。</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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<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="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <math.h></span>
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<span class="co2">#include <float.h></span>
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<span class="co2">#include <errno.h></span>
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<span class="co2">#include <fenv.h></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="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(7, -4) = %f<span class="es1">\n</span>"</span>, ldexp<span class="br0">(</span><span class="nu0">7</span>, <span class="sy2">-</span><span class="nu0">4</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(1, -1074) = %g (minimum positive subnormal double)<span class="es1">\n</span>"</span>,
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ldexp<span class="br0">(</span><span class="nu0">1</span>, <span class="sy2">-</span><span class="nu0">1074</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(nextafter(1,0), 1024) = %g (largest finite double)<span class="es1">\n</span>"</span>,
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ldexp<span class="br0">(</span><a href="c-numeric-math-nexttoward.html"><span class="kw700">nextafter</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="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 特殊值</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(-0, 10) = %f<span class="es1">\n</span>"</span>, ldexp<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="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(-Inf, -1) = %f<span class="es1">\n</span>"</span>, ldexp<span class="br0">(</span><span class="sy2">-</span>INFINITY, <span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 错误处理</span>
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<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="kw717">feclearexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw728">FE_ALL_EXCEPT</span></a><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw850">printf</span></a><span class="br0">(</span><span class="st0">"ldexp(1, 1024) = %f<span class="es1">\n</span>"</span>, ldexp<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">1024</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<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="kw878">perror</span></a><span class="br0">(</span><span class="st0">" errno == ERANGE"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span><a href="c-numeric-fenv-fetestexcept.html"><span class="kw718">fetestexcept</span></a><span class="br0">(</span><a href="c-numeric-fenv-FE_exceptions.html"><span class="kw732">FE_OVERFLOW</span></a><span class="br0">)</span><span class="br0">)</span> <a href="c-io-puts.html"><span class="kw835">puts</span></a><span class="br0">(</span><span class="st0">" FE_OVERFLOW raised"</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>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">ldexp(7, -4) = 0.437500
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ldexp(1, -1074) = 4.94066e-324 (minimum positive subnormal double)
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ldexp(nextafter(1,0), 1024) = 1.79769e+308 (largest finite double)
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ldexp(-0, 10) = -0.000000
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ldexp(-Inf, -1) = -inf
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ldexp(1, 1024) = inf
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errno == ERANGE: Numerical result out of range
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FE_OVERFLOW raised</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.12.6.6 The ldexp functions (p: 244)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>7.25 Type-generic math <tgmath.h> (p: 373-375)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>F.10.3.6 The ldexp functions (p: 522)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.12.6.6 The ldexp functions (p: 225)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>7.22 Type-generic math <tgmath.h> (p: 335-337)</li>
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</ul>
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</dd>
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</dl>
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<dl>
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<dd>
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<ul>
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<li>F.9.3.6 The ldexp functions (p: 459)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.5.4.3 The ldexp function</li>
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</ul>
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</dd>
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</dl>
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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="c-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-c99">(C99)</span></span><span><span class="t-mark-rev t-since-c99">(C99)</span></span></span></div>
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</div>
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</td>
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<td>将数拆分成有效数字和 <span class="t-c"><span class="mw-geshi c source-c"><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="c-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>
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<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><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>
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</div>
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</td>
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<td>高效计算一个数乘 <span class="t-lc"><a href="c-types-limits.html">FLT_RADIX</a></span> 的幂<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>
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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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</body>
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