193 lines
16 KiB
HTML
193 lines
16 KiB
HTML
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<title>std::modf, std::modff, std::modfl</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>modf, <span style="font-size:0.7em; line-height:130%">std::</span>modff, <span style="font-size:0.7em; line-height:130%">std::</span>modfl</h1>
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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>定义于头文件 <code><a href="cpp-header-cmath.html"><cmath></a></code></div>
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<td></td>
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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> modf <span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">float</span><span class="sy2">*</span> iptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">float</span> modff<span class="br0">(</span> <span class="kw4">float</span> x, <span class="kw4">float</span><span class="sy2">*</span> iptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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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="kw4">double</span> modf <span class="br0">(</span> <span class="kw4">double</span> x, <span class="kw4">double</span><span class="sy2">*</span> iptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td rowspan="142">(3)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> modf <span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">long</span> <span class="kw4">double</span><span class="sy2">*</span> iptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">long</span> <span class="kw4">double</span> modfl<span class="br0">(</span> <span class="kw4">long</span> <span class="kw4">double</span> x, <span class="kw4">long</span> <span class="kw4">double</span><span class="sy2">*</span> iptr <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div class="t-li1"><span class="t-li">1-3)</span> 分解给定的浮点值 <code>arg</code> 为整数和分数部分，每个都拥有与 <code>arg</code> 相同的类型和符号。（以浮点格式）存储整数部分于 <code>iptr</code> 所指向的对象。</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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<td>arg</td>
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<td>-</td>
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<td>浮点值</td>
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<tr class="t-par">
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<td>iptr</td>
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<td>-</td>
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<td>指向要存储整数部分的目标的浮点值的指针</td>
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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，则返回与 <code>x</code> 相同符号的 <code>x</code> 小数部分。将整数部分放进 <code>iptr</code> 所指向的值。</p>
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<p>返回值和存储于 <code>*iptr</code> 的值的和给出 <code>arg</code> （允许舍入）。</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>此函数不受制于任何指定于 <span class="t-lc"><a href="cpp-numeric-math-math_errhandling.html">math_errhandling</a></span> 的错误。</p>
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<p>若实现支持 IEE 浮点算术（ IEC 60559 ），则</p>
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<ul>
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<li>若 <code>arg</code> 为 ±0 ，则返回 ±0 ，并存储 ±0 于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>iptr</span></span> 。</li>
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<li>若 <code>arg</code> 为 ±∞ ，则返回 ±0 ，并存储 ±∞ 于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>iptr</span></span> 。</li>
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<li>若 <code>arg</code> 为 NaN ，则返回 NaN ，并存储 NaN 于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>iptr</span></span> 。</li>
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<li>返回值是准确的，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>此函数表现为如同实现如下：</p>
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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="kw4">double</span> modf<span class="br0">(</span><span class="kw4">double</span> x, <span class="kw4">double</span><span class="sy2">*</span> iptr<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co2">#pragma STDC FENV_ACCESS ON</span>
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<span class="kw4">int</span> save_round <span class="sy1">=</span> <a href="cpp-numeric-fenv-feround.html"><span class="kw1454">std::<span class="me2">fegetround</span></span></a><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-numeric-fenv-feround.html"><span class="kw1455">std::<span class="me2">fesetround</span></span></a><span class="br0">(</span><a href="cpp-numeric-fenv-FE_round.html"><span class="kw1468">FE_TOWARDZERO</span></a><span class="br0">)</span><span class="sy4">;</span>
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<span class="sy2">*</span>iptr <span class="sy1">=</span> <a href="cpp-numeric-math-nearbyint.html"><span class="kw1356">std::<span class="me2">nearbyint</span></span></a><span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-numeric-fenv-feround.html"><span class="kw1455">std::<span class="me2">fesetround</span></span></a><span class="br0">(</span>save_round<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="cpp-numeric-math-copysign.html"><span class="kw1366">std::<span class="me2">copysign</span></span></a><span class="br0">(</span><a href="cpp-numeric-math-isinf.html"><span class="kw1369">std::<span class="me2">isinf</span></span></a><span class="br0">(</span>x<span class="br0">)</span> <span class="sy4">?</span> <span class="nu16">0.0</span> <span class="sy4">:</span> x <span class="sy2">-</span> <span class="br0">(</span><span class="sy2">*</span>iptr<span class="br0">)</span>, x<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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<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="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> std<span class="sy4">::</span><span class="me2">modf</span><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="kw1360">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> <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>
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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> <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>
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<span class="co1">// 特殊值</span>
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f2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">modf</span><span class="br0">(</span><span class="sy2">-</span><span class="nu16">0.0</span>, <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(-0) 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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f2 <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">modf</span><span class="br0">(</span><span class="sy2">-</span><a href="cpp-numeric-math-INFINITY.html"><span class="kw1376">INFINITY</span></a>, <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(-Inf) 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="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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modf(-0) makes -0 + -0
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modf(-Inf) makes -INF + -0</pre></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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<div class="t-dsc-member-div">
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<div><a href="cpp-numeric-math-trunc.html"><span class="t-lines"><span>trunc</span><span>truncf</span><span>truncl</span></span></a></div>
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<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>
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<td>绝对值不大于给定值的最接近整数<br />
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<span class="t-mark">(函数)</span></td>
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</table>
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<div class="visualClear"></div>
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