307 lines
20 KiB
Plaintext
Generated
307 lines
20 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: Reading Files</title>
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<link rel=stylesheet href="site.css">
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</head>
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<script>
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window.onkeydown = (e) => {
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return;
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window.location.href = 'base64-encoding';
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if (e.key == "ArrowRight") {
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window.location.href = 'writing-files';
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<body>
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<div class="example" id="reading-files">
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<h2><a href="./">Go by Example</a>: Reading Files</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Reading and writing files are basic tasks needed for
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many Go programs. First we’ll look at some examples of
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reading files.</p>
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</td>
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<td class="code empty leading">
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="https://go.dev/play/p/SKTzfpnV0To"><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 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">"io"</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="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>Reading files requires checking most calls for errors.
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This helper will streamline our error checks below.</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="kd">func</span> <span class="nf">check</span><span class="p">(</span><span class="nx">e</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="k">if</span> <span class="nx">e</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="nb">panic</span><span class="p">(</span><span class="nx">e</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 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>Perhaps the most basic file reading task is
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slurping a file’s entire contents into memory.</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">dat</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">ReadFile</span><span class="p">(</span><span class="s">"/tmp/dat"</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">fmt</span><span class="p">.</span><span class="nf">Print</span><span class="p">(</span><span class="nb">string</span><span class="p">(</span><span class="nx">dat</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>You’ll often want more control over how and what
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parts of a file are read. For these tasks, start
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by <code>Open</code>ing a file to obtain an <code>os.File</code> value.</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">f</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">Open</span><span class="p">(</span><span class="s">"/tmp/dat"</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</span><span class="p">(</span><span class="nx">err</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>Read some bytes from the beginning of the file.
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Allow up to 5 to be read but also note how many
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actually were read.</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">b1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">n1</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Read</span><span class="p">(</span><span class="nx">b1</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes: %s\n"</span><span class="p">,</span> <span class="nx">n1</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b1</span><span class="p">[:</span><span class="nx">n1</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>You can also <code>Seek</code> to a known location in the file
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and <code>Read</code> from there.</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">o2</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="nx">io</span><span class="p">.</span><span class="nx">SeekStart</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">b2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">n2</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Read</span><span class="p">(</span><span class="nx">b2</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes @ %d: "</span><span class="p">,</span> <span class="nx">n2</span><span class="p">,</span> <span class="nx">o2</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">Printf</span><span class="p">(</span><span class="s">"%v\n"</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b2</span><span class="p">[:</span><span class="nx">n2</span><span class="p">]))</span></span></span></code></pre>
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</td>
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</tr>
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<td class="docs">
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<p>Other methods of seeking are relative to the
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current cursor position,</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">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="mi">2</span><span class="p">,</span> <span class="nx">io</span><span class="p">.</span><span class="nx">SeekCurrent</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</span><span class="p">(</span><span class="nx">err</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>and relative to the end of the file.</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">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="o">-</span><span class="mi">4</span><span class="p">,</span> <span class="nx">io</span><span class="p">.</span><span class="nx">SeekEnd</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</span><span class="p">(</span><span class="nx">err</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>The <code>io</code> package provides some functions that may
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be helpful for file reading. For example, reads
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like the ones above can be more robustly
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implemented with <code>ReadAtLeast</code>.</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">o3</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="nx">io</span><span class="p">.</span><span class="nx">SeekStart</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">b3</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">n3</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">io</span><span class="p">.</span><span class="nf">ReadAtLeast</span><span class="p">(</span><span class="nx">f</span><span class="p">,</span> <span class="nx">b3</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes @ %d: %s\n"</span><span class="p">,</span> <span class="nx">n3</span><span class="p">,</span> <span class="nx">o3</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b3</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>There is no built-in rewind, but
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<code>Seek(0, io.SeekStart)</code> accomplishes this.</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">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="nx">io</span><span class="p">.</span><span class="nx">SeekStart</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</span><span class="p">(</span><span class="nx">err</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>The <code>bufio</code> package implements a buffered
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reader that may be useful both for its efficiency
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with many small reads and because of the additional
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reading methods it provides.</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">r4</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nf">NewReader</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">b4</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">r4</span><span class="p">.</span><span class="nf">Peek</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nf">check</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">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"5 bytes: %s\n"</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b4</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>Close the file when you’re done (usually this would
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be scheduled immediately after <code>Open</code>ing with
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<code>defer</code>).</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="nx">f</span><span class="p">.</span><span class="nf">Close</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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</table>
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<table>
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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="gp">$</span> echo "hello" > /tmp/dat
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</span></span><span class="line"><span class="cl"><span class="gp">$</span> echo "go" >> /tmp/dat
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</span></span><span class="line"><span class="cl"><span class="gp">$</span> go run reading-files.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">go
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</span></span></span><span class="line"><span class="cl"><span class="go">5 bytes: hello
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</span></span></span><span class="line"><span class="cl"><span class="go">2 bytes @ 6: go
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</span></span></span><span class="line"><span class="cl"><span class="go">2 bytes @ 6: go
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</span></span></span><span class="line"><span class="cl"><span class="go">5 bytes: hello</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>Next we’ll look at writing files.</p>
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</td>
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<td class="code empty">
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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="writing-files" rel="next">Writing Files</a>.
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</p>
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<p class="footer">
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Based on
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<em><a href="https://gobyexample.com/">Go by Example</a></em>
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by <a href="https://markmcgranaghan.com">Mark McGranaghan</a>
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and <a href="https://eli.thegreenplace.net">Eli Bendersky</a>,
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licensed under
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<a href="https://creativecommons.org/licenses/by/3.0/">CC BY 3.0</a>.
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<a href="https://git.inkch.xyz/inkch/gobyexample/commit/de3b5db09c9b980ba971c708a3fe522606e44572">
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Style modifications by inkch
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</a>.
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<a href="https://github.com/mmcgrana/gobyexample">Original source</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 \"io\"\u000A \"os\"\u000A)\u000A');codeLines.push('func check(e error) {\u000A if e !\u003D nil {\u000A panic(e)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' dat, err :\u003D os.ReadFile(\"/tmp/dat\")\u000A check(err)\u000A fmt.Print(string(dat))\u000A');codeLines.push(' f, err :\u003D os.Open(\"/tmp/dat\")\u000A check(err)\u000A');codeLines.push(' b1 :\u003D make([]byte, 5)\u000A n1, err :\u003D f.Read(b1)\u000A check(err)\u000A fmt.Printf(\"%d bytes: %s\\n\", n1, string(b1[:n1]))\u000A');codeLines.push(' o2, err :\u003D f.Seek(6, io.SeekStart)\u000A check(err)\u000A b2 :\u003D make([]byte, 2)\u000A n2, err :\u003D f.Read(b2)\u000A check(err)\u000A fmt.Printf(\"%d bytes @ %d: \", n2, o2)\u000A fmt.Printf(\"%v\\n\", string(b2[:n2]))\u000A');codeLines.push(' _, err \u003D f.Seek(2, io.SeekCurrent)\u000A check(err)\u000A');codeLines.push(' _, err \u003D f.Seek(-4, io.SeekEnd)\u000A check(err)\u000A');codeLines.push(' o3, err :\u003D f.Seek(6, io.SeekStart)\u000A check(err)\u000A b3 :\u003D make([]byte, 2)\u000A n3, err :\u003D io.ReadAtLeast(f, b3, 2)\u000A check(err)\u000A fmt.Printf(\"%d bytes @ %d: %s\\n\", n3, o3, string(b3))\u000A');codeLines.push(' _, err \u003D f.Seek(0, io.SeekStart)\u000A check(err)\u000A');codeLines.push(' r4 :\u003D bufio.NewReader(f)\u000A b4, err :\u003D r4.Peek(5)\u000A check(err)\u000A fmt.Printf(\"5 bytes: %s\\n\", string(b4))\u000A');codeLines.push(' f.Close()\u000A}\u000A');codeLines.push('');codeLines.push('');
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