gobyexample/public/reading-files

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      <h2><a href="./">Go в примерах</a>: Чтение файлов (Reading Files)</h2>

      <table>

        <tr>
          <td class="docs">
            <p>Чтение и запись файлов это базовая задача, необходимая
для решения множества задач. Для начала мы рассмотрим
несколько примеров чтения файлов.</p>

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          <td class="code empty leading">


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          <td class="docs">

          </td>
          <td class="code leading">
            <a href="http://play.golang.org/p/m6mvXlZtHTX" target="_blank"><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>

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        </tr>

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          <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>

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          <td class="docs">
            <p>Чтение файлов требует проверок множества вызовов на
наличие ошибок. Эта функция-хелпер поможет нам
обрабатывать ошибки в одном месте.</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>

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          <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>

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        </tr>

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          <td class="docs">
            <p>Возможно, самая основная задача чтения файлов -
это сохранение всего содержимого файла в памяти.</p>

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          <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>

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          <td class="docs">
            <p>Вам часто может потребоваться больший контроль
над тем, как и какие части файла читаются. Для
решения этих задач начните с открытия файла,
чтобы получить значение <code>os.File</code>..</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>

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          <td class="docs">
            <p>Прочитаем несколько байт с начала файла. Будет
прочитано первые 5 байт, но также выведем,
сколько фактически было прочитано.</p>

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          <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>

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        </tr>

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          <td class="docs">
            <p>Вы так же можете получить конкретное место файла
с помощью <code>Seek</code> и выполнить <code>Read</code> оттуда.</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>

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        <tr>
          <td class="docs">
            <p>Пакет <code>io</code> предоставляет некоторые функции, которые
могут быть полезны для чтения файлов. Например,
чтение, подобное приведенному выше, может быть
более надежно реализовано с помощью <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>

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        <tr>
          <td class="docs">
            <p>Тут нет встроенной перемотки назад, но можно
использовать <code>Seek(0, 0)</code> для этого.</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>

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        </tr>

        <tr>
          <td class="docs">
            <p>В пакете <code>bufio</code> реализован буферизованный ридер,
который может быть полезен из-за своей эффективности
при большом количестве небольших операций чтения, и
из-за наличия дополнительных методов чтения, которые
он предоставляет.</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>

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        <tr>
          <td class="docs">
            <p>Закройте файл, когда вы закончите использовать его
(обычно закрытие с <code>defer</code>&lsquo;ом делается сразу после открытия).</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>

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          <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>

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          <td class="docs">
            <p>Далее рассмотрим запись в файл.</p>

          </td>
          <td class="code empty">


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        </tr>

      </table>


      <p class="next">
        Следующий пример: <a href="writing-files">Запись файлов (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>
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