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<li><em>Go语言圣经读书笔记(挖坑中...): <a href="https://github.com/chai2010/gopl-notes-zh">https://github.com/chai2010/gopl-notes-zh</a></em></li>
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<h2 id="32-浮点数"><a class="header" href="#32-浮点数">3.2. 浮点数</a></h2>
<p>Go语言提供了两种精度的浮点数float32和float64。它们的算术规范由IEEE754浮点数国际标准定义该浮点数规范被所有现代的CPU支持。</p>
<p>这些浮点数类型的取值范围可以从很微小到很巨大。浮点数的范围极限值可以在math包找到。常量math.MaxFloat32表示float32能表示的最大数值大约是 3.4e38对应的math.MaxFloat64常量大约是1.8e308。它们分别能表示的最小值近似为1.4e-45和4.9e-324。</p>
<p>一个float32类型的浮点数可以提供大约6个十进制数的精度而float64则可以提供约15个十进制数的精度通常应该优先使用float64类型因为float32类型的累计计算误差很容易扩散并且float32能精确表示的正整数并不是很大译注因为float32的有效bit位只有23个其它的bit位用于指数和符号当整数大于23bit能表达的范围时float32的表示将出现误差</p>
<pre><code class="language-Go">var f float32 = 16777216 // 1 &lt;&lt; 24
fmt.Println(f == f+1) // &quot;true&quot;!
</code></pre>
<p>浮点数的字面值可以直接写小数部分,像这样:</p>
<pre><code class="language-Go">const e = 2.71828 // (approximately)
</code></pre>
<p>小数点前面或后面的数字都可能被省略(例如.707或1.。很小或很大的数最好用科学计数法书写通过e或E来指定指数部分</p>
<pre><code class="language-Go">const Avogadro = 6.02214129e23 // 阿伏伽德罗常数
const Planck = 6.62606957e-34 // 普朗克常数
</code></pre>
<p>用Printf函数的%g参数打印浮点数将采用更紧凑的表示形式打印并提供足够的精度但是对应表格的数据使用%e带指数或%f的形式打印可能更合适。所有的这三个打印形式都可以指定打印的宽度和控制打印精度。</p>
<pre><code class="language-Go">for x := 0; x &lt; 8; x++ {
fmt.Printf(&quot;x = %d e^x = %8.3f\n&quot;, x, math.Exp(float64(x)))
}
</code></pre>
<p>上面代码打印e的幂打印精度是小数点后三个小数精度和8个字符宽度</p>
<pre><code>x = 0 e^x = 1.000
x = 1 e^x = 2.718
x = 2 e^x = 7.389
x = 3 e^x = 20.086
x = 4 e^x = 54.598
x = 5 e^x = 148.413
x = 6 e^x = 403.429
x = 7 e^x = 1096.633
</code></pre>
<p>math包中除了提供大量常用的数学函数外还提供了IEEE754浮点数标准中定义的特殊值的创建和测试正无穷大和负无穷大分别用于表示太大溢出的数字和除零的结果还有NaN非数一般用于表示无效的除法操作结果0/0或Sqrt(-1).</p>
<pre><code class="language-Go">var z float64
fmt.Println(z, -z, 1/z, -1/z, z/z) // &quot;0 -0 +Inf -Inf NaN&quot;
</code></pre>
<p>函数math.IsNaN用于测试一个数是否是非数NaNmath.NaN则返回非数对应的值。虽然可以用math.NaN来表示一个非法的结果但是测试一个结果是否是非数NaN则是充满风险的因为NaN和任何数都是不相等的译注在浮点数中NaN、正无穷大和负无穷大都不是唯一的每个都有非常多种的bit模式表示</p>
<pre><code class="language-Go">nan := math.NaN()
fmt.Println(nan == nan, nan &lt; nan, nan &gt; nan) // &quot;false false false&quot;
</code></pre>
<p>如果一个函数返回的浮点数结果可能失败,最好的做法是用单独的标志报告失败,像这样:</p>
<pre><code class="language-Go">func compute() (value float64, ok bool) {
// ...
if failed {
return 0, false
}
return result, true
}
</code></pre>
<p>接下来的程序演示了通过浮点计算生成的图形。它是带有两个参数的z = f(x, y)函数的三维形式使用了可缩放矢量图形SVG格式输出SVG是一个用于矢量线绘制的XML标准。图3.1显示了sin(r)/r函数的输出图形其中r是<code>sqrt(x*x+y*y)</code></p>
<p><img src="../images/ch3-01.png" alt="" /></p>
<p><u><i>gopl.io/ch3/surface</i></u></p>
<pre><code class="language-Go">// Surface computes an SVG rendering of a 3-D surface function.
package main
import (
&quot;fmt&quot;
&quot;math&quot;
)
const (
width, height = 600, 320 // canvas size in pixels
cells = 100 // number of grid cells
xyrange = 30.0 // axis ranges (-xyrange..+xyrange)
xyscale = width / 2 / xyrange // pixels per x or y unit
zscale = height * 0.4 // pixels per z unit
angle = math.Pi / 6 // angle of x, y axes (=30°)
)
var sin30, cos30 = math.Sin(angle), math.Cos(angle) // sin(30°), cos(30°)
func main() {
fmt.Printf(&quot;&lt;svg xmlns='http://www.w3.org/2000/svg' &quot;+
&quot;style='stroke: grey; fill: white; stroke-width: 0.7' &quot;+
&quot;width='%d' height='%d'&gt;&quot;, width, height)
for i := 0; i &lt; cells; i++ {
for j := 0; j &lt; cells; j++ {
ax, ay := corner(i+1, j)
bx, by := corner(i, j)
cx, cy := corner(i, j+1)
dx, dy := corner(i+1, j+1)
fmt.Printf(&quot;&lt;polygon points='%g,%g %g,%g %g,%g %g,%g'/&gt;\n&quot;,
ax, ay, bx, by, cx, cy, dx, dy)
}
}
fmt.Println(&quot;&lt;/svg&gt;&quot;)
}
func corner(i, j int) (float64, float64) {
// Find point (x,y) at corner of cell (i,j).
x := xyrange * (float64(i)/cells - 0.5)
y := xyrange * (float64(j)/cells - 0.5)
// Compute surface height z.
z := f(x, y)
// Project (x,y,z) isometrically onto 2-D SVG canvas (sx,sy).
sx := width/2 + (x-y)*cos30*xyscale
sy := height/2 + (x+y)*sin30*xyscale - z*zscale
return sx, sy
}
func f(x, y float64) float64 {
r := math.Hypot(x, y) // distance from (0,0)
return math.Sin(r) / r
}
</code></pre>
<p>要注意的是corner函数返回了两个结果分别对应每个网格顶点的坐标参数。</p>
<p>要解释这个程序是如何工作的需要一些基本的几何学知识但是我们可以跳过几何学原理因为程序的重点是演示浮点数运算。程序的本质是三个不同的坐标系中映射关系如图3.2所示。第一个是100x100的二维网格对应整数坐标(i,j),从远处的(0,0)位置开始。我们从远处向前面绘制,因此远处先绘制的多边形有可能被前面后绘制的多边形覆盖。</p>
<p>第二个坐标系是一个三维的网格浮点坐标(x,y,z)其中x和y是i和j的线性函数通过平移转换为网格单元的中心然后用xyrange系数缩放。高度z是函数f(x,y)的值。</p>
<p>第三个坐标系是一个二维的画布,起点(0,0)在左上角。画布中点的坐标用(sx,sy)表示。我们使用等角投影将三维点(x,y,z)投影到二维的画布中。</p>
<p><img src="../images/ch3-02.png" alt="" /></p>
<p>画布中从远处到右边的点对应较大的x值和较大的y值。并且画布中x和y值越大则对应的z值越小。x和y的垂直和水平缩放系数来自30度角的正弦和余弦值。z的缩放系数0.4,是一个任意选择的参数。</p>
<p>对于二维网格中的每一个网格单元main函数计算单元的四个顶点在画布中对应多边形ABCD的顶点其中B对应(i,j)顶点位置A、C和D是其它相邻的顶点然后输出SVG的绘制指令。</p>
<p><strong>练习 3.1</strong> 如果f函数返回的是无限制的float64值那么SVG文件可能输出无效的<polygon>多边形元素虽然许多SVG渲染器会妥善处理这类问题。修改程序跳过无效的多边形。</p>
<p><strong>练习 3.2</strong> 试验math包中其他函数的渲染图形。你是否能输出一个egg box、moguls或a saddle图案?</p>
<p><strong>练习 3.3</strong> 根据高度给每个多边形上色,那样峰值部将是红色(#ff0000谷部将是蓝色#0000ff</p>
<p><strong>练习 3.4</strong> 参考1.7节Lissajous例子的函数构造一个web服务器用于计算函数曲面然后返回SVG数据给客户端。服务器必须设置Content-Type头部</p>
<pre><code class="language-Go">w.Header().Set(&quot;Content-Type&quot;, &quot;image/svg+xml&quot;)
</code></pre>
<p>这一步在Lissajous例子中不是必须的因为服务器使用标准的PNG图像格式可以根据前面的512个字节自动输出对应的头部。允许客户端通过HTTP请求参数设置高度、宽度和颜色等参数。</p>
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