yeray/gobyexample
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
gobyexample/public/reading-files
Mark McGranaghan a1fd3bf037 Regenerate
2020-04-10 17:49:18 -07:00

295 lines
15 KiB
Plaintext
Generated
Raw Blame History

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Reading Files</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'base64-encoding';
}
if (e.key == "ArrowRight") {
window.location.href = 'writing-files';
}
}
</script>
<body>
<div class="example" id="reading-files">
<h2><a href="./">Go by Example</a>: Reading Files</h2>
<table>
<tr>
<td class="docs">
<p>Reading and writing files are basic tasks needed for
many Go programs. First we&rsquo;ll look at some examples of
reading files.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/kF0cDC0drsX"><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">&quot;bufio&quot;</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;io&quot;</span>
<span class="s">&quot;io/ioutil&quot;</span>
<span class="s">&quot;os&quot;</span>
<span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Reading files requires checking most calls for errors.
This helper will streamline our error checks below.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">check</span><span class="p">(</span><span class="nx">e</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nb">panic</span><span class="p">(</span><span class="nx">e</span><span class="p">)</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="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>Perhaps the most basic file reading task is
slurping a file&rsquo;s entire contents into memory.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">dat</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">ioutil</span><span class="p">.</span><span class="nx">ReadFile</span><span class="p">(</span><span class="s">&quot;/tmp/dat&quot;</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nb">string</span><span class="p">(</span><span class="nx">dat</span><span class="p">))</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>You&rsquo;ll often want more control over how and what
parts of a file are read. For these tasks, start
by <code>Open</code>ing a file to obtain an <code>os.File</code> value.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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="nx">Open</span><span class="p">(</span><span class="s">&quot;/tmp/dat&quot;</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Read some bytes from the beginning of the file.
Allow up to 5 to be read but also note how many
actually were read.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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>
<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="nx">Read</span><span class="p">(</span><span class="nx">b1</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%d bytes: %s\n&quot;</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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>You can also <code>Seek</code> to a known location in the file
and <code>Read</code> from there.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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="nx">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<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>
<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="nx">Read</span><span class="p">(</span><span class="nx">b2</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%d bytes @ %d: &quot;</span><span class="p">,</span> <span class="nx">n2</span><span class="p">,</span> <span class="nx">o2</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%v\n&quot;</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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>The <code>io</code> package provides some functions that may
be helpful for file reading. For example, reads
like the ones above can be more robustly
implemented with <code>ReadAtLeast</code>.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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="nx">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<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>
<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="nx">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>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%d bytes @ %d: %s\n&quot;</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>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>There is no built-in rewind, but <code>Seek(0, 0)</code>
accomplishes this.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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="nx">Seek</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>The <code>bufio</code> package implements a buffered
reader that may be useful both for its efficiency
with many small reads and because of the additional
reading methods it provides.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">r4</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nx">NewReader</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<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="nx">Peek</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;5 bytes: %s\n&quot;</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b4</span><span class="p">))</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Close the file when you&rsquo;re done (usually this would
be scheduled immediately after <code>Open</code>ing with
<code>defer</code>).</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="nx">f</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> <span class="nb">echo</span> <span class="s2">&quot;hello&quot;</span> &gt; /tmp/dat
<span class="gp">$</span> <span class="nb">echo</span> <span class="s2">&quot;go&quot;</span> &gt;&gt; /tmp/dat
<span class="gp">$</span> go run reading-files.go
<span class="go">hello</span>
<span class="go">go</span>
<span class="go">5 bytes: hello</span>
<span class="go">2 bytes @ 6: go</span>
<span class="go">2 bytes @ 6: go</span>
<span class="go">5 bytes: hello</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Next we&rsquo;ll look at writing files.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="writing-files">Writing Files</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/reading-files">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 \"io\"\u000A \"io/ioutil\"\u000A \"os\"\u000A)\u000A');codeLines.push('func check(e error) {\u000A if e !\x3D nil {\u000A panic(e)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' dat, err :\x3D ioutil.ReadFile(\"/tmp/dat\")\u000A check(err)\u000A fmt.Print(string(dat))\u000A');codeLines.push(' f, err :\x3D os.Open(\"/tmp/dat\")\u000A check(err)\u000A');codeLines.push(' b1 :\x3D make([]byte, 5)\u000A n1, err :\x3D f.Read(b1)\u000A check(err)\u000A fmt.Printf(\"%d bytes: %s\\n\", n1, string(b1[:n1]))\u000A');codeLines.push(' o2, err :\x3D f.Seek(6, 0)\u000A check(err)\u000A b2 :\x3D make([]byte, 2)\u000A n2, err :\x3D 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(' o3, err :\x3D f.Seek(6, 0)\u000A check(err)\u000A b3 :\x3D make([]byte, 2)\u000A n3, err :\x3D io.ReadAtLeast(f, b3, 2)\u000A check(err)\u000A fmt.Printf(\"%d bytes @ %d: %s\\n\", n3, o3, string(b3))\u000A');codeLines.push(' _, err \x3D f.Seek(0, 0)\u000A check(err)\u000A');codeLines.push(' r4 :\x3D bufio.NewReader(f)\u000A b4, err :\x3D 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('');
</script>
<script src="site.js" async></script>
</body>
</html>
Reference in New Issue View Git Blame Copy Permalink