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<?xml version="1.0" encoding="utf-8"?>
<rss xmlns:atom="http://www.w3.org/2005/Atom" version="2.0">
<channel>
<title>ywdxz</title>
<link>http://ywdxz.github.io/</link>
<description>一天天的</description>
<generator>Hugo 0.79.0 https://gohugo.io/</generator>
<language>zh-CN</language>
<managingEditor>[email protected] (xz)</managingEditor>
<webMaster>[email protected] (xz)</webMaster>
<copyright>[CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh)</copyright>
<lastBuildDate>Fri, 11 Dec 2020 14:53:37 +0800</lastBuildDate>
<atom:link rel="self" type="application/rss+xml" href="http://ywdxz.github.io/rss.xml" />
<item>
<title>关于math.NaN() 问题记录</title>
<link>http://ywdxz.github.io/posts/2020-12-11-3/</link>
<guid isPermaLink="true">http://ywdxz.github.io/posts/2020-12-11-3/</guid>
<pubDate>Fri, 11 Dec 2020 14:40:49 +0800</pubDate>
<author>[email protected] (xz)</author>
<copyright>[CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh)</copyright>
<description><h2 id="问题">问题</h2>
<p>看 map 源码看到一个测试case,<strong>为什么 k == k</strong></p>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre class="chroma"><code><span class="lnt"> 1
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<td class="lntd">
<pre class="chroma"><code class="language-go" data-lang="go"><span class="kd">func</span> <span class="nf">testMapNan</span><span class="p">(</span><span class="nx">t</span> <span class="o">*</span><span class="nx">testing</span><span class="p">.</span><span class="nx">T</span><span class="p">,</span> <span class="nx">m</span> <span class="kd">map</span><span class="p">[</span><span class="kt">float64</span><span class="p">]</span><span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="nx">m</span><span class="p">)</span> <span class="o">!=</span> <span class="mi">3</span> <span class="p">{</span>
<span class="nx">t</span><span class="p">.</span><span class="nf">Error</span><span class="p">(</span><span class="s">&#34;length wrong&#34;</span><span class="p">)</span>
<span class="p">}</span>
<span class="nx">s</span> <span class="o">:=</span> <span class="mi">0</span>
<span class="k">for</span> <span class="nx">k</span><span class="p">,</span> <span class="nx">v</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">m</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">k</span> <span class="o">==</span> <span class="nx">k</span> <span class="p">{</span> <span class="c1">// why ?????
</span><span class="c1"></span> <span class="nx">t</span><span class="p">.</span><span class="nf">Error</span><span class="p">(</span><span class="s">&#34;nan disappeared&#34;</span><span class="p">)</span>
<span class="p">}</span>
<span class="k">if</span> <span class="p">(</span><span class="nx">v</span> <span class="o">&amp;</span> <span class="p">(</span><span class="nx">v</span> <span class="o">-</span> <span class="mi">1</span><span class="p">))</span> <span class="o">!=</span> <span class="mi">0</span> <span class="p">{</span>
<span class="nx">t</span><span class="p">.</span><span class="nf">Error</span><span class="p">(</span><span class="s">&#34;value wrong&#34;</span><span class="p">)</span>
<span class="p">}</span>
<span class="nx">s</span> <span class="o">|=</span> <span class="nx">v</span>
<span class="p">}</span>
<span class="k">if</span> <span class="nx">s</span> <span class="o">!=</span> <span class="mi">7</span> <span class="p">{</span>
<span class="nx">t</span><span class="p">.</span><span class="nf">Error</span><span class="p">(</span><span class="s">&#34;values wrong&#34;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="c1">// nan is a good test because nan != nan, and nan has
</span><span class="c1">// a randomized hash value.
</span><span class="c1"></span><span class="kd">func</span> <span class="nf">TestMapAssignmentNan</span><span class="p">(</span><span class="nx">t</span> <span class="o">*</span><span class="nx">testing</span><span class="p">.</span><span class="nx">T</span><span class="p">)</span> <span class="p">{</span>
<span class="nx">m</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">(</span><span class="kd">map</span><span class="p">[</span><span class="kt">float64</span><span class="p">]</span><span class="kt">int</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nx">nan</span> <span class="o">:=</span> <span class="nx">math</span><span class="p">.</span><span class="nf">NaN</span><span class="p">()</span>
<span class="c1">// Test assignment.
</span><span class="c1"></span> <span class="nx">m</span><span class="p">[</span><span class="nx">nan</span><span class="p">]</span> <span class="p">=</span> <span class="mi">1</span>
<span class="nx">m</span><span class="p">[</span><span class="nx">nan</span><span class="p">]</span> <span class="p">=</span> <span class="mi">2</span>
<span class="nx">m</span><span class="p">[</span><span class="nx">nan</span><span class="p">]</span> <span class="p">=</span> <span class="mi">4</span>
<span class="nf">testMapNan</span><span class="p">(</span><span class="nx">t</span><span class="p">,</span> <span class="nx">m</span><span class="p">)</span>
<span class="p">}</span>
</code></pre></td></tr></table>
</div>
</div><h2 id="结论">结论</h2>
<p>eg:</p>
<pre><code>func main() {
m := make(map[float64]int)
m[1.4] = 1
m[2.4] = 2
m[math.NaN()] = 3
m[math.NaN()] = 3
for k, v := range m {
fmt.Printf(&quot;[%v, %d] &quot;, k, v)
}
fmt.Printf(&quot;\nk: %v, v: %d\n&quot;, math.NaN(), m[math.NaN()])
fmt.Printf(&quot;k: %v, v: %d\n&quot;, 2.400000000001, m[2.400000000001])
fmt.Printf(&quot;k: %v, v: %d\n&quot;, 2.4000000000000000000000001, m[2.4000000000000000000000001])
fmt.Println(math.NaN() == math.NaN())
}
</code></pre><p>输出：</p>
<pre><code>[2.4, 2] [NaN, 3] [NaN, 3] [1.4, 1]
k: NaN, v: 0
k: 2.400000000001, v: 0
k: 2.4, v: 2
false
</code></pre><p>结论：
NaN 不等于 NaN,<strong>为什么NaN!=NaN呢？</strong></p>
</description>
</item>
<item>
<title>环境变量整理</title>
<link>http://ywdxz.github.io/posts/2020-12-11-1/</link>
<guid isPermaLink="true">http://ywdxz.github.io/posts/2020-12-11-1/</guid>
<pubDate>Fri, 11 Dec 2020 11:38:25 +0800</pubDate>
<author>[email protected] (xz)</author>
<copyright>[CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh)</copyright>
<description><p>经常会用到需要手工设置linux环境变量的情况，每次都是google一下，这边把结果整理一下作为知识储备。</p>
<h3 id="临时设置">临时设置</h3>
<ul>
<li>直接执行 export GOBACTRACE=crash： 仅对当前shell生效</li>
<li>env GOBACTRACE=crash</li>
<li>set GOBACTRACE=crash</li>
</ul>
<p><strong>set、env、export</strong></p>
<ul>
<li>set：显示/设置 shell变量，包括私有变量和用户变量</li>
<li>env：显示/设置 用户变量</li>
<li>export：显示/设置 当前导出成用户变量的shell变量</li>
</ul>
<p>linux变量分为shell变量和用户变量，export是一个命令，可以将shell变量导出成用户变量。这些变量都可以通过unset来清除掉。</p>
<h3 id="永久设置">永久设置</h3>
<p>在如下的文件中加上 export GOBACTRACE=crash，然后source一下该文件就可以永久生效。一般选项用bash_profile或者bashrc</p>
<ul>
<li>/etch/profile：对全部用户生效</li>
<li>~/.bash_profile：对当前用户生效</li>
<li>~/.bashrc：启动新的shell是读取并执行</li>
<li>~/.bash_login：shell登录是读取并执行</li>
<li>~/.bash_logout：shell退出登录时读取并执行</li>
</ul>
<p>shell的初始化顺序如下：</p>
<ul>
<li>\1. 执行 /etc/profile 的命令，如果存在的话</li>
<li>\2. 执行 ~/.bash_profile 的命令，如果存在的话</li>
<li>\3. 执行 ~/.bash_login 的命令，如果存在的话</li>
<li>\4. 执行 ~/.bashrc 的命令，如果存在的话</li>
</ul>
<p>ps：通过在如上的文件中设置一些命令，可以在用户登录时就执行一些操作，如cd到指定目录</p>
</description>
</item>
<item>
<title>浮点数整理</title>
<link>http://ywdxz.github.io/posts/2020-12-11-2/</link>
<guid isPermaLink="true">http://ywdxz.github.io/posts/2020-12-11-2/</guid>
<pubDate>Mon, 30 Nov 2020 23:18:10 +0800</pubDate>
<author>[email protected] (xz)</author>
<copyright>[CC BY-NC-SA 4.0](https://creativecommons.org/licenses/by-nc-sa/4.0/deed.zh)</copyright>
<description><h1 id="浮点数整理">浮点数整理</h1>
<h2 id="标准">标准</h2>
<p>根据国际标准IEEE 754，任意一个二进制浮点数可以表示成下面科学计数的形式：</p>
<pre><code>value = (-1)^sign * fraction * 2^exponent
</code></pre><h2 id="内存布局">内存布局</h2>
<div class="highlight"><div class="chroma">
<table class="lntable"><tr><td class="lntd">
<pre class="chroma"><code><span class="lnt">1
</span><span class="lnt">2
</span><span class="lnt">3
</span></code></pre></td>
<td class="lntd">
<pre class="chroma"><code class="language-go" data-lang="go"> <span class="o">-----------------------</span>
<span class="p">|</span> <span class="nx">sign</span><span class="p">|</span><span class="nx">exponent</span><span class="p">|</span><span class="nx">fraction</span><span class="p">|</span>
<span class="o">-----------------------</span>
</code></pre></td></tr></table>
</div>
</div><h2 id="sign">sign</h2>
<p>符号位，1 byte.（ 0 表示正数，1 表示负数）</p>
<h2 id="exponent">exponent</h2>
<p>指数位，单精度 8 byte，双精度 23 byte</p>
<p>exponent 是一个无符号整数，但 exponent 需要表示负数。所以IEEE 754规定，exponent 的真实值必须再减去一个中间数。对于单精度，中间数为：<strong>127</strong>；对于双精度，中间数为：<strong>1023</strong>。</p>
<p><strong>Top</strong>:</p>
<ol>
<li>exponent 二进制全为 1 时：fraction 全为 0，表示无穷大（±INF），否则表示不是一个数 （NaN）</li>
<li>exponent 二进制全为 0 时：exponent 真实值为（1-127/1-1023），fraction xxxxx 表示的真实值为 0.xxxxx.（方便表示±0，以及接近于0很小的数字）</li>
</ol>
<h2 id="fraction">fraction</h2>
<p>有效数字，单精度 11 byte，双精度 52 byte</p>
<p>（1 &lt;= fraction &lt; 2 ），默认这个数总是 1.xxxx 的形式，因此1可以舍去，只保存后面的xxxxxx部分</p>
<p><strong>Top</strong>: exponent 二进制全为 0 时,默认这个数总为 0.xxxx 的形式，因此0可以舍去，只保存后面的xxxxxx部分</p>
<h2 id="十进制小数转换为二进制小数">十进制小数转换为二进制小数</h2>
<p> 十进制小数转换成二进制小数采用&quot;乘2取整，顺序排列&quot;法。</p>
<p> 具体做法是：用2乘十进制小数，可以得到积，将积的整数部分取出，再用2乘余下的小数 部分，又得到一个积，再将积的整数部分取出，如此进行，直到积中的小数部分为零，或者达到所要求的精度为止。</p>
<p>eg： 把（0.8125）转换为二进制小数</p>
<p>0.8125 *2 = 1.625 ==&gt; 1</p>
<p>0.625 * 2 = 1.25 ==&gt; 1</p>
<p>0.25 * 2 = 0.5 ==&gt; 0</p>
<p>0.5 * 2 = 1 ==&gt; 1</p>
<p>所以：（0.8125）10 ==&gt;（0.1101）2</p>
<h2 id="浮点数编码演示">浮点数编码演示</h2>
<p>eg: float32(0.5)</p>
<p>sign = 0</p>
<p>exponent = 127</p>
<p>fraction = 1</p>
</description>
</item>
</channel>
</rss>