194 lines
8.6 KiB
Plaintext
Generated
194 lines
8.6 KiB
Plaintext
Generated
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Line Filters</title>
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<div class="example" id="line-filters">
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<h2><a href="./">Go by Example</a>: Line Filters</h2>
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<table>
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<tr>
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<td class="docs">
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<p>A <em>line filter</em> is a common type of program that reads
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input on stdin, processes it, and then prints some
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derived result to stdout. <code>grep</code> and <code>sed</code> are common
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line filters.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Here’s an example line filter in Go that writes a
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capitalized version of all input text. You can use this
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pattern to write your own Go line filters.</p>
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</td>
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<td class="code leading">
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<a href="https://go.dev/play/p/kNcupWRsYPP"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"bufio"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"fmt"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"os"</span>
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</span></span><span class="line"><span class="cl"> <span class="s">"strings"</span>
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</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Wrapping the unbuffered <code>os.Stdin</code> with a buffered
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scanner gives us a convenient <code>Scan</code> method that
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advances the scanner to the next token; which is
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the next line in the default scanner.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">scanner</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nf">NewScanner</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stdin</span><span class="p">)</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p><code>Text</code> returns the current token, here the next line,
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from the input.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="k">for</span> <span class="nx">scanner</span><span class="p">.</span><span class="nf">Scan</span><span class="p">()</span> <span class="p">{</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">ucl</span> <span class="o">:=</span> <span class="nx">strings</span><span class="p">.</span><span class="nf">ToUpper</span><span class="p">(</span><span class="nx">scanner</span><span class="p">.</span><span class="nf">Text</span><span class="p">())</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Write out the uppercased line.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">ucl</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Check for errors during <code>Scan</code>. End of file is
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expected and not reported by <code>Scan</code> as an error.</p>
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</td>
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<td class="code">
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<pre class="chroma"><code><span class="line"><span class="cl"> <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="nf">Err</span><span class="p">();</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">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>
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</span></span><span class="line"><span class="cl"> <span class="nx">os</span><span class="p">.</span><span class="nf">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="p">}</span>
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</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>To try out our line filter, first make a file with a few
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lowercase lines.</p>
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</td>
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<td class="code leading">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> echo 'hello' > /tmp/lines
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</span></span><span class="line"><span class="cl"><span class="gp">$</span> echo 'filter' >> /tmp/lines</span></span></code></pre>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Then use the line filter to get uppercase lines.</p>
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</td>
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<td class="code">
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<pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> cat /tmp/lines | go run line-filters.go
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</span></span><span class="line"><span class="cl"><span class="go">HELLO
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</span></span></span><span class="line"><span class="cl"><span class="go">FILTER</span></span></span></code></pre>
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="file-paths">File Paths</a>.
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</p>
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<p class="footer">
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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</p>
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</div>
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<script>
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var codeLines = [];
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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 :\u003D bufio.NewScanner(os.Stdin)\u000A');codeLines.push(' for scanner.Scan() {\u000A');codeLines.push(' ucl :\u003D strings.ToUpper(scanner.Text())\u000A');codeLines.push(' fmt.Println(ucl)\u000A }\u000A');codeLines.push(' if err :\u003D scanner.Err(); err !\u003D nil {\u000A fmt.Fprintln(os.Stderr, \"error:\", err)\u000A os.Exit(1)\u000A }\u000A}\u000A');codeLines.push('');codeLines.push('');
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