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<div class="example" id="line-filters">
<h2><a href="./">Go by Example</a>: Line Filters</h2>
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<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>
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<p>Here&rsquo;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>
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<a href="http://play.golang.org/p/mpYwOHj2ma"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;bufio&quot;</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;os&quot;</span>
<span class="s">&quot;strings&quot;</span>
<span class="p">)</span>
</pre></div>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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<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>
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<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>
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<p><code>Text</code> returns the current token, here the next line,
from the input.</p>
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<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>
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<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>
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<p>Write out the uppercased line.</p>
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<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>
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<p>Check for errors during <code>Scan</code>. End of file is
expected and not reported by <code>Scan</code> as an error.</p>
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<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">&quot;error:&quot;</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>
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<p>To try out our line filter, first make a file with a few
lowercase lines.</p>
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<div class="highlight"><pre><span class="gp">$</span> <span class="nb">echo</span> <span class="s1">&#39;hello&#39;</span> &gt; /tmp/lines
<span class="gp">$</span> <span class="nb">echo</span> <span class="s1">&#39;filter&#39;</span> &gt;&gt; /tmp/lines
</pre></div>
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<p>Then use the line filter to get uppercase lines.</p>
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<div class="highlight"><pre><span class="gp">$</span> cat /tmp/lines | go run line-filters.go
<span class="go">HELLO</span>
<span class="go">FILTER</span>
</pre></div>
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<p class="next">
Next example: <a href="command-line-arguments">Command-Line Arguments</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</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>
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