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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Line Filters</title>
<link rel=stylesheet href="site.css">
</head>
<script>
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<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" />
<pre class="chroma">
<span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><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>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</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">
<pre class="chroma">
<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>
</pre>
</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">
<pre class="chroma"> <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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"> <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>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Write out the uppercased line.</p>
</td>
<td class="code leading">
<pre class="chroma">
<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>
<span class="p">}</span>
</pre>
</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">
<pre class="chroma">
<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>
<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>
<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>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</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">
<pre class="chroma">
<span class="gp">$</span> echo 'hello' > /tmp/lines
<span class="gp">$</span> echo 'filter' >> /tmp/lines</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Then use the line filter to get uppercase lines.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="gp">$</span> cat /tmp/lines | go run line-filters.go
<span class="go">HELLO
</span><span class="go">FILTER</span></pre>
</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> 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>
</p>
</div>
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