74 lines
6.2 KiB
HTML
74 lines
6.2 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>std::random_device::entropy</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_numeric_random_random_device_entropy skin-cppreference2 action-view cpp-navbar">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::random_device::</span>entropy</h1>
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<div id="bodyContent">
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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-random.html">random</a>‎ | <a href="cpp-numeric-random-random_device.html">random device</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">double</span> entropy<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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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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<p>获得随机数设备熵的估计，它是 0 与 <span class="texhtml" style="white-space: nowrap;">log<span class="t-su t-su-b"><br />
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2</span>(max()+1)</span> （等于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><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">unsigned</span> <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">digits</span></span></span> ）之间的浮点数。若设备拥有 <span class="texhtml" style="white-space: nowrap;">n</span> 个状态，而其概率各为 <span class="texhtml" style="white-space: nowrap;">P<span class="t-su t-su-b"><br />
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0</span>,...,P<span class="t-su t-su-b"><br />
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n-1</span></span> ，则设备熵 <span class="texhtml" style="white-space: nowrap;">S</span> 定义为</p>
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<p><span class="texhtml" style="white-space: nowrap;">S = -Σ<span class="t-su t-su-b">n-1<br />
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i=0</span>P<span class="t-su t-su-b"><br />
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i</span>log(P<span class="t-su t-su-b"><br />
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i</span>)</span></p>
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<p>确定的随机数生成器（例如伪随机数生成器）拥有零熵。</p>
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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>设备熵的值，或若不可应用则为零。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>此函数在一些标准库中未完全实现。例如，版本 12 前的 <a rel="nofollow" class="external text" href="https://github.com/llvm-mirror/libcxx/blob/master/src/random.cpp#L174">LLVM libc++</a> 始终返回零，即使设备是非确定的（对于 libstdc++ ，见 <a rel="nofollow" class="external text" href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67578">bug 67578</a> ）。与之相比， Microsoft Visual C++ 实现始终返回 32 ，且 <a rel="nofollow" class="external text" href="https://github.com/boostorg/random/blob/master/src/random_device.cpp#L242">boost.random</a> 返回 10 。</p>
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<p>Linux 内核设备 /dev/urandom 的熵可用 <a rel="nofollow" class="external text" href="http://man7.org/linux/man-pages/man4/random.4.html">ioctl RNDGETENTCNT</a> 获得—— <a rel="nofollow" class="external text" href="https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B%2B11/random.cc#L188">GNU libstdc++</a> 从版本 8.1 起将它用于 <code>std::random_device::entropy()</code> 。</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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<p>实现之一上的示例输出</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 <random></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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<a href="cpp-numeric-random-random_device.html"><span class="kw1486">std::<span class="me2">random_device</span></span></a> rd<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> rd.<span class="me1">entropy</span><span class="br0">(</span><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="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">32</pre></div>
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