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<h1 id="firstHeading" class="firstHeading"><small>FE_DOWNWARD, FE_TONEAREST, FE_TOWARDZERO, FE_UPWARD</small></h1>
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<div id="contentSub"><span class="subpages">&lt; <a href="cpp.html">cpp</a>&lrm; | <a href="cpp-numeric.html">numeric</a>&lrm; | <a href="cpp-numeric-fenv.html">fenv</a></span></div>
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<table class="t-dcl-begin">
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<div>&#23450;&#20041;&#20110;&#22836;&#25991;&#20214; <code><a href="cpp-header-cfenv.html">&lt;cfenv&gt;</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="co2">#define FE_DOWNWARD &nbsp; &nbsp; /*implementation defined*/</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
</tr>
<tr class="t-dcl t-since-cxx11">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="co2">#define FE_TONEAREST &nbsp; &nbsp;/*implementation defined*/</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
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<tr class="t-dcl t-since-cxx11">
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<div><span class="mw-geshi cpp source-cpp"><span class="co2">#define FE_TOWARDZERO &nbsp; /*implementation defined*/</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
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<tr class="t-dcl t-since-cxx11">
<td>
<div><span class="mw-geshi cpp source-cpp"><span class="co2">#define FE_UPWARD &nbsp; &nbsp; &nbsp; /*implementation defined*/</span></span></div>
</td>
<td class="t-dcl-nopad"></td>
<td><span class="t-mark-rev t-since-cxx11">(C++11 &#36215;)</span></td>
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<td></td>
<td></td>
<td></td>
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<p>&#27599;&#20010;&#36825;&#31181;&#23439;&#23637;&#24320;&#25104;&#38750;&#36127;&#24120;&#37327;&#34920;&#36798;&#24335;&#65292;&#21487;&#34987; <span class="t-lc"><a href="cpp-numeric-fenv-feround.html">std::fesetround</a></span> &#25110; <span class="t-lc"><a href="cpp-numeric-fenv-feround.html">std::fegetround</a></span> &#29992;&#20110;&#25351;&#31034;&#21463;&#25903;&#25345;&#30340;&#28014;&#28857;&#33293;&#20837;&#27169;&#24335;&#20043;&#19968;&#12290;&#23454;&#29616;&#21487;&#20110; <code>&lt;cfenv&gt;</code> &#23450;&#20041;&#21478;&#22806;&#30340;&#33293;&#20837;&#27169;&#24335;&#24120;&#37327;&#65292;&#23427;&#24212;&#20197; <code>FE_</code> &#21518;&#38543;&#33267;&#23569;&#19968;&#20010;&#22823;&#20889;&#23383;&#27597;&#24320;&#22987;&#12290;&#27599;&#20010;&#23439;&#20165;&#33509;&#21463;&#25903;&#25345;&#25165;&#24471;&#21040;&#23450;&#20041;&#12290;</p>
<p>&#22823;&#22810;&#25968;&#23454;&#29616;&#20013;&#65292;&#36825;&#20123;&#23439;&#24120;&#37327;&#23637;&#24320;&#25104;&#30340;&#20540;&#31561;&#20110; <span class="t-lc"><a href="cpp-types-climits-FLT_ROUNDS.html">FLT_ROUNDS</a></span> &#21644; <span class="t-lc"><a href="cpp-types-numeric_limits-float_round_style.html">std::float_round_style</a></span> &#30340;&#20540;&#12290;</p>
<table class="t-dsc-begin">
<tr class="t-dsc-hitem">
<td>&#24120;&#37327;</td>
<td>&#35299;&#37322;</td>
</tr>
<tr class="t-dsc">
<td><code>FE_DOWNWARD</code></td>
<td>&#21521;&#36127;&#26080;&#31351;&#22823;&#33293;&#20837;</td>
</tr>
<tr class="t-dsc">
<td><code>FE_TONEAREST</code></td>
<td>&#21521;&#26368;&#25509;&#36817;&#21487;&#34920;&#31034;&#20540;&#33293;&#20837;</td>
</tr>
<tr class="t-dsc">
<td><code>FE_TOWARDZERO</code></td>
<td>&#21521;&#38646;&#33293;&#20837;</td>
</tr>
<tr class="t-dsc">
<td><code>FE_UPWARD</code></td>
<td>&#21521;&#27491;&#26080;&#31351;&#22823;&#33293;&#20837;</td>
</tr>
</table>
<p>&#23454;&#29616;&#21487;&#25903;&#25345;&#21478;&#22806;&#30340;&#33293;&#20837;&#27169;&#24335;&#12290;</p>
<p>&#24403;&#21069;&#33293;&#20837;&#27169;&#24335;&#24433;&#21709;&#19979;&#21015;&#20869;&#23481;&#65306;</p>
<ul>
<li>&#24120;&#37327;&#34920;&#36798;&#24335;&#22806;&#28014;&#28857;<a href="cpp-language-operator_arithmetic.html">&#31639;&#26415;&#36816;&#31639;&#31526;</a>&#30340;&#32467;&#26524;</li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="kw4">double</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
x<span class="sy2">/</span><span class="nu0">10</span><span class="sy4">;</span> <span class="co1">// 0.09999999999999999167332731531132594682276248931884765625</span>
<span class="co1">// or 0.1000000000000000055511151231257827021181583404541015625</span></pre></div>
</div>
<ul>
<li>&#26631;&#20934;&#24211;<a href="cpp-numeric-math.html">&#25968;&#23398;&#20989;&#25968;</a>&#30340;&#32467;&#26524;</li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><a href="cpp-numeric-math-sqrt.html"><span class="kw1329">std::<span class="me2">sqrt</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 1.41421356237309492343001693370752036571502685546875</span>
<span class="co1">// or 1.4142135623730951454746218587388284504413604736328125</span></pre></div>
</div>
<ul>
<li>&#28014;&#28857;&#21040;&#28014;&#28857;&#38544;&#24335;&#36716;&#25442;&#25110;&#36716;&#22411;</li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><span class="kw4">double</span> d <span class="sy1">=</span> <span class="nu0">1</span> <span class="sy2">+</span> <a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1">&lt;</span><span class="kw4">double</span><span class="sy1">&gt;</span><span class="sy4">::</span><span class="me2">epsilon</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">float</span> f <span class="sy1">=</span> d<span class="sy4">;</span> <span class="co1">// 1.00000000000000000000000</span>
<span class="co1">// or 1.00000011920928955078125</span></pre></div>
</div>
<ul>
<li>&#20363;&#22914; <span class="t-lc"><a href="cpp-string-byte-strtof.html">std::strtod</a></span> &#25110; <span class="t-lc"><a href="cpp-io-c-fprintf.html">std::printf</a></span> &#30340;&#23383;&#31526;&#20018;&#36716;&#25442;</li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><a href="cpp-string-basic_string-stof.html"><span class="kw1262">std::<span class="me2">stof</span></span></a><span class="br0">(</span><span class="st0">&quot;0.1&quot;</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 0.0999999940395355224609375</span>
<span class="co1">// or 0.100000001490116119384765625</span></pre></div>
</div>
<ul>
<li>&#24211;&#33293;&#20837;&#20989;&#25968; <span class="t-lc"><a href="cpp-numeric-math-nearbyint.html">std::nearbyint</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-rint.html">std::rint</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-rint.html">std::lrint</a></span></li>
</ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="cpp source-cpp">
<pre class="de1"><a href="cpp-numeric-math-rint.html"><span class="kw1358">std::<span class="me2">lrint</span></span></a><span class="br0">(</span><span class="nu16">2.1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 2 &#25110; 3</span></pre></div>
</div>
<p>&#24403;&#21069;&#33293;&#20837;&#27169;&#24335;&#19981;&#24433;&#21709;&#65306;</p>
<ul>
<li>&#28014;&#28857;&#21040;&#25972;&#25968;&#30340;&#38544;&#24335;&#36716;&#25442;&#25110;&#36716;&#22411;&#65288;&#22987;&#32456;&#21521;&#38646;&#65289;</li>
<li>&#32534;&#35793;&#26102;&#25191;&#34892;&#30340;&#24120;&#37327;&#34920;&#36798;&#24335;&#20013;&#28014;&#28857;&#31639;&#26415;&#36816;&#31639;&#31526;&#30340;&#32467;&#26524;&#65288;&#22987;&#32456;&#21521;&#26368;&#25509;&#36817;&#65289;</li>
<li>&#24211;&#20989;&#25968; <span class="t-lc"><a href="cpp-numeric-math-round.html">std::round</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-round.html">std::lround</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-round.html">std::llround</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-ceil.html">std::ceil</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-floor.html">std::floor</a></span> &#12289; <span class="t-lc"><a href="cpp-numeric-math-trunc.html">std::trunc</a></span></li>
</ul>
<p>&#21516;&#20219;&#20309;<a href="cpp-numeric-fenv.html">&#28014;&#28857;&#29615;&#22659;</a>&#21151;&#33021;&#65292;&#20165;&#33509;&#35774;&#32622;&#20102; <code>#pragma STDC FENV_ACCESS ON</code> &#25165;&#20445;&#35777;&#33293;&#20837;&#12290;</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">&#31034;&#20363;</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 &lt;iostream&gt;</span>
<span class="co2">#include &lt;iomanip&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;cfenv&gt;</span>
<span class="co2">#include &lt;cmath&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
<span class="co2">#pragma STDC FENV_ACCESS ON</span>
<a href="cpp-numeric-fenv-feround.html"><span class="kw1456">std::<span class="me2">fesetround</span></span></a><span class="br0">(</span>FE_DOWNWARD<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1754">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding down: <span class="es1">\n</span>&quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-io-manip-setprecision.html"><span class="kw1792">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">50</span><span class="br0">)</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; pi = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-numeric-math-acos.html"><span class="kw1337">std::<span class="me2">acos</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span>.<span class="me1">f</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot;stof(<span class="es1">\&quot;</span>1.1<span class="es1">\&quot;</span>) = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-string-basic_string-stof.html"><span class="kw1262">std::<span class="me2">stof</span></span></a><span class="br0">(</span><span class="st0">&quot;1.1&quot;</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; rint(2.1) = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-numeric-math-rint.html"><span class="kw1357">std::<span class="me2">rint</span></span></a><span class="br0">(</span><span class="nu16">2.3</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;<span class="es1">\n</span><span class="es1">\n</span>&quot;</span><span class="sy4">;</span>
<a href="cpp-numeric-fenv-feround.html"><span class="kw1456">std::<span class="me2">fesetround</span></span></a><span class="br0">(</span>FE_UPWARD<span class="br0">)</span><span class="sy4">;</span>
<a href="cpp-io-cout.html"><span class="kw1754">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">&quot;rounding up: <span class="es1">\n</span>&quot;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; pi = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-numeric-math-acos.html"><span class="kw1337">std::<span class="me2">acos</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span>.<span class="me1">f</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot;stof(<span class="es1">\&quot;</span>1.1<span class="es1">\&quot;</span>) = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-string-basic_string-stof.html"><span class="kw1262">std::<span class="me2">stof</span></span></a><span class="br0">(</span><span class="st0">&quot;1.1&quot;</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span>
<span class="sy1">&lt;&lt;</span> <span class="st0">&quot; rint(2.1) = &quot;</span> <span class="sy1">&lt;&lt;</span> <a href="cpp-numeric-math-rint.html"><span class="kw1357">std::<span class="me2">rint</span></span></a><span class="br0">(</span><span class="nu16">2.3</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">&#39;<span class="es1">\n</span>&#39;</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div>
</div>
<p>&#36755;&#20986;&#65306;</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;">
<div class="text source-text">
<pre class="de1">rounding down:
pi = 3.141592502593994140625
stof(&quot;1.1&quot;) = 1.099999904632568359375
rint(2.1) = 2
rounding up:
pi = 3.1415927410125732421875
stof(&quot;1.1&quot;) = 1.10000002384185791015625
rint(2.1) = 3</pre></div>
</div>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">&#21442;&#38405;</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-types-numeric_limits-float_round_style.html"><span class="t-lines"><span>float_round_style</span></span></a></div>
</div>
</td>
<td>&#25351;&#31034;&#28014;&#28857;&#33293;&#20837;&#27169;&#24335;<br />
<span class="t-mark">(&#26522;&#20030;)</span></td>
</tr>
<tr class="t-dsc">
<td>
<div class="t-dsc-member-div">
<div><a href="cpp-numeric-fenv-feround.html"><span class="t-lines"><span>fegetround</span><span>fesetround</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></div>
</div>
</td>
<td>&#33719;&#21462;&#25110;&#35774;&#32622;&#33293;&#20837;&#26041;&#21521;<br />
<span class="t-mark">(&#20989;&#25968;)</span></td>
</tr>
</table>
</div>
<div class="visualClear"></div>
</div>
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