212 lines
7.6 KiB
Plaintext
Generated
212 lines
7.6 KiB
Plaintext
Generated
<!DOCTYPE html>
|
|
<html>
|
|
<head>
|
|
<meta charset="utf-8">
|
|
<title>Go by Example: Line Filters</title>
|
|
<link rel=stylesheet href="site.css">
|
|
</head>
|
|
<script>
|
|
onkeydown = (e) => {
|
|
|
|
if (e.key == "ArrowLeft") {
|
|
window.location.href = 'writing-files';
|
|
}
|
|
|
|
|
|
if (e.key == "ArrowRight") {
|
|
window.location.href = 'file-paths';
|
|
}
|
|
|
|
}
|
|
</script>
|
|
<body>
|
|
<div class="example" id="line-filters">
|
|
<h2><a href="./">Go by Example</a>: Line Filters</h2>
|
|
|
|
<table>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>A <em>line filter</em> is a common type of program that reads
|
|
input on stdin, processes it, and then prints some
|
|
derived result to stdout. <code>grep</code> and <code>sed</code> are common
|
|
line filters.</p>
|
|
|
|
</td>
|
|
<td class="code empty leading">
|
|
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Here’s an example line filter in Go that writes a
|
|
capitalized version of all input text. You can use this
|
|
pattern to write your own Go line filters.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
<a href="http://play.golang.org/p/kNcupWRsYPP"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
|
|
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
|
|
<span class="s">"bufio"</span>
|
|
<span class="s">"fmt"</span>
|
|
<span class="s">"os"</span>
|
|
<span class="s">"strings"</span>
|
|
<span class="p">)</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Wrapping the unbuffered <code>os.Stdin</code> with a buffered
|
|
scanner gives us a convenient <code>Scan</code> method that
|
|
advances the scanner to the next token; which is
|
|
the next line in the default scanner.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre> <span class="nx">scanner</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nx">NewScanner</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stdin</span><span class="p">)</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p><code>Text</code> returns the current token, here the next line,
|
|
from the input.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre> <span class="k">for</span> <span class="nx">scanner</span><span class="p">.</span><span class="nx">Scan</span><span class="p">()</span> <span class="p">{</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre> <span class="nx">ucl</span> <span class="o">:=</span> <span class="nx">strings</span><span class="p">.</span><span class="nx">ToUpper</span><span class="p">(</span><span class="nx">scanner</span><span class="p">.</span><span class="nx">Text</span><span class="p">())</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Write out the uppercased line.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">ucl</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Check for errors during <code>Scan</code>. End of file is
|
|
expected and not reported by <code>Scan</code> as an error.</p>
|
|
|
|
</td>
|
|
<td class="code">
|
|
|
|
<div class="highlight"><pre> <span class="k">if</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">scanner</span><span class="p">.</span><span class="nx">Err</span><span class="p">();</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
|
|
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintln</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">"error:"</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
|
|
<span class="nx">os</span><span class="p">.</span><span class="nx">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
|
|
<span class="p">}</span>
|
|
<span class="p">}</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
</table>
|
|
|
|
<table>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>To try out our line filter, first make a file with a few
|
|
lowercase lines.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<div class="highlight"><pre><span class="gp">$</span> <span class="nb">echo</span> <span class="s1">'hello'</span> > /tmp/lines
|
|
<span class="gp">$</span> <span class="nb">echo</span> <span class="s1">'filter'</span> >> /tmp/lines
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Then use the line filter to get uppercase lines.</p>
|
|
|
|
</td>
|
|
<td class="code">
|
|
|
|
<div class="highlight"><pre><span class="gp">$</span> cat /tmp/lines <span class="p">|</span> go run line-filters.go
|
|
<span class="go">HELLO</span>
|
|
<span class="go">FILTER</span>
|
|
</pre></div>
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
</table>
|
|
|
|
|
|
<p class="next">
|
|
Next example: <a href="file-paths">File Paths</a>.
|
|
</p>
|
|
|
|
<p class="footer">
|
|
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/line-filters">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
|
|
</p>
|
|
</div>
|
|
<script>
|
|
var codeLines = [];
|
|
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"bufio\"\u000A \"fmt\"\u000A \"os\"\u000A \"strings\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' scanner :\x3D bufio.NewScanner(os.Stdin)\u000A');codeLines.push(' for scanner.Scan() {\u000A');codeLines.push(' ucl :\x3D strings.ToUpper(scanner.Text())\u000A');codeLines.push(' fmt.Println(ucl)\u000A }\u000A');codeLines.push(' if err :\x3D scanner.Err(); err !\x3D nil {\u000A fmt.Fprintln(os.Stderr, \"error:\", err)\u000A os.Exit(1)\u000A }\u000A}\u000A');codeLines.push('');codeLines.push('');
|
|
</script>
|
|
<script src="site.js" async></script>
|
|
</body>
|
|
</html>
|