327 lines
21 KiB
Plaintext
Generated
327 lines
21 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: Slices</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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onkeydown = (e) => {
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if (e.key == "ArrowLeft") {
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window.location.href = 'arrays';
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if (e.key == "ArrowRight") {
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window.location.href = 'maps';
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}
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</script>
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<body>
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<div class="example" id="slices">
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<h2><a href="./">Go by Example</a>: Slices</h2>
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<table>
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<tr>
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<td class="docs">
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<p><em>Slices</em> are an important data type in Go, giving
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a more powerful interface to sequences than arrays.</p>
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</td>
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<td class="code empty leading">
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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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</td>
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<td class="code leading">
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<a href="https://go.dev/play/p/kiy1StxorBF"><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>
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</tr>
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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="kn">import</span> <span class="p">(</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">"slices"</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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<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>Unlike arrays, slices are typed only by the
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elements they contain (not the number of elements).
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An uninitialized slice equals to nil and has
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length 0.</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">var</span> <span class="nx">s</span> <span class="p">[]</span><span class="kt">string</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">Println</span><span class="p">(</span><span class="s">"uninit:"</span><span class="p">,</span> <span class="nx">s</span><span class="p">,</span> <span class="nx">s</span> <span class="o">==</span> <span class="kc">nil</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span> <span class="o">==</span> <span class="mi">0</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>To create an empty slice with non-zero length, use
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the builtin <code>make</code>. Here we make a slice of
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<code>string</code>s of length <code>3</code> (initially zero-valued).
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By default a new slice’s capacity is equal to its
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length; if we know the slice is going to grow ahead
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of time, it’s possible to pass a capacity explicitly
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as an additional parameter to <code>make</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">s</span> <span class="p">=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">string</span><span class="p">,</span> <span class="mi">3</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">Println</span><span class="p">(</span><span class="s">"emp:"</span><span class="p">,</span> <span class="nx">s</span><span class="p">,</span> <span class="s">"len:"</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">),</span> <span class="s">"cap:"</span><span class="p">,</span> <span class="nb">cap</span><span class="p">(</span><span class="nx">s</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>We can set and get just like with arrays.</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">s</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="p">=</span> <span class="s">"a"</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">s</span><span class="p">[</span><span class="mi">1</span><span class="p">]</span> <span class="p">=</span> <span class="s">"b"</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">s</span><span class="p">[</span><span class="mi">2</span><span class="p">]</span> <span class="p">=</span> <span class="s">"c"</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">Println</span><span class="p">(</span><span class="s">"set:"</span><span class="p">,</span> <span class="nx">s</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">Println</span><span class="p">(</span><span class="s">"get:"</span><span class="p">,</span> <span class="nx">s</span><span class="p">[</span><span class="mi">2</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><code>len</code> returns the length of the slice as expected.</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">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"len:"</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</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>In addition to these basic operations, slices
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support several more that make them richer than
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arrays. One is the builtin <code>append</code>, which
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returns a slice containing one or more new values.
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Note that we need to accept a return value from
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<code>append</code> as we may get a new slice 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">s</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">s</span><span class="p">,</span> <span class="s">"d"</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">s</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">s</span><span class="p">,</span> <span class="s">"e"</span><span class="p">,</span> <span class="s">"f"</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">Println</span><span class="p">(</span><span class="s">"apd:"</span><span class="p">,</span> <span class="nx">s</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>Slices can also be <code>copy</code>’d. Here we create an
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empty slice <code>c</code> of the same length as <code>s</code> and copy
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into <code>c</code> from <code>s</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">c</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">string</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">s</span><span class="p">))</span>
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</span></span><span class="line"><span class="cl"> <span class="nb">copy</span><span class="p">(</span><span class="nx">c</span><span class="p">,</span> <span class="nx">s</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">Println</span><span class="p">(</span><span class="s">"cpy:"</span><span class="p">,</span> <span class="nx">c</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>Slices support a “slice” operator with the syntax
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<code>slice[low:high]</code>. For example, this gets a slice
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of the elements <code>s[2]</code>, <code>s[3]</code>, and <code>s[4]</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">l</span> <span class="o">:=</span> <span class="nx">s</span><span class="p">[</span><span class="mi">2</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">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"sl1:"</span><span class="p">,</span> <span class="nx">l</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>This slices up to (but excluding) <code>s[5]</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">l</span> <span class="p">=</span> <span class="nx">s</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">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"sl2:"</span><span class="p">,</span> <span class="nx">l</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 this slices up from (and including) <code>s[2]</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">l</span> <span class="p">=</span> <span class="nx">s</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">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"sl3:"</span><span class="p">,</span> <span class="nx">l</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>We can declare and initialize a variable for slice
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in a single line as well.</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">t</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">string</span><span class="p">{</span><span class="s">"g"</span><span class="p">,</span> <span class="s">"h"</span><span class="p">,</span> <span class="s">"i"</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">Println</span><span class="p">(</span><span class="s">"dcl:"</span><span class="p">,</span> <span class="nx">t</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>slices</code> package contains a number of useful
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utility functions for slices.</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">t2</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">string</span><span class="p">{</span><span class="s">"g"</span><span class="p">,</span> <span class="s">"h"</span><span class="p">,</span> <span class="s">"i"</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">slices</span><span class="p">.</span><span class="nf">Equal</span><span class="p">(</span><span class="nx">t</span><span class="p">,</span> <span class="nx">t2</span><span class="p">)</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">Println</span><span class="p">(</span><span class="s">"t == t2"</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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<tr>
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<td class="docs">
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<p>Slices can be composed into multi-dimensional data
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structures. The length of the inner slices can
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vary, unlike with multi-dimensional arrays.</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">twoD</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([][]</span><span class="kt">int</span><span class="p">,</span> <span class="mi">3</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="k">for</span> <span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">i</span> <span class="p"><</span> <span class="mi">3</span><span class="p">;</span> <span class="nx">i</span><span class="o">++</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">innerLen</span> <span class="o">:=</span> <span class="nx">i</span> <span class="o">+</span> <span class="mi">1</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">twoD</span><span class="p">[</span><span class="nx">i</span><span class="p">]</span> <span class="p">=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">int</span><span class="p">,</span> <span class="nx">innerLen</span><span class="p">)</span>
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</span></span><span class="line"><span class="cl"> <span class="k">for</span> <span class="nx">j</span> <span class="o">:=</span> <span class="mi">0</span><span class="p">;</span> <span class="nx">j</span> <span class="p"><</span> <span class="nx">innerLen</span><span class="p">;</span> <span class="nx">j</span><span class="o">++</span> <span class="p">{</span>
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</span></span><span class="line"><span class="cl"> <span class="nx">twoD</span><span class="p">[</span><span class="nx">i</span><span class="p">][</span><span class="nx">j</span><span class="p">]</span> <span class="p">=</span> <span class="nx">i</span> <span class="o">+</span> <span class="nx">j</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>
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</span></span><span class="line"><span class="cl"> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"2d: "</span><span class="p">,</span> <span class="nx">twoD</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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<p>Note that while slices are different types than arrays,
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they are rendered similarly by <code>fmt.Println</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="gp">$</span> go run slices.go
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</span></span><span class="line"><span class="cl"><span class="go">uninit: [] true true
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</span></span></span><span class="line"><span class="cl"><span class="go">emp: [ ] len: 3 cap: 3
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</span></span></span><span class="line"><span class="cl"><span class="go">set: [a b c]
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</span></span></span><span class="line"><span class="cl"><span class="go">get: c
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</span></span></span><span class="line"><span class="cl"><span class="go">len: 3
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</span></span></span><span class="line"><span class="cl"><span class="go">apd: [a b c d e f]
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</span></span></span><span class="line"><span class="cl"><span class="go">cpy: [a b c d e f]
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</span></span></span><span class="line"><span class="cl"><span class="go">sl1: [c d e]
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</span></span></span><span class="line"><span class="cl"><span class="go">sl2: [a b c d e]
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</span></span></span><span class="line"><span class="cl"><span class="go">sl3: [c d e f]
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</span></span></span><span class="line"><span class="cl"><span class="go">dcl: [g h i]
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</span></span></span><span class="line"><span class="cl"><span class="go">t == t2
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</span></span></span><span class="line"><span class="cl"><span class="go">2d: [[0] [1 2] [2 3 4]]</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>Check out this <a href="https://go.dev/blog/slices-intro">great blog post</a>
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by the Go team for more details on the design and
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implementation of slices in Go.</p>
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</td>
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<td class="code empty leading">
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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>Now that we’ve seen arrays and slices we’ll look at
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Go’s other key builtin data structure: maps.</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="maps">Maps</a>.
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</p>
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<p class="footer">
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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>
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</p>
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</div>
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"slices\"\u000A)\u000A');codeLines.push('func main() {\u000A');codeLines.push(' var s []string\u000A fmt.Println(\"uninit:\", s, s \u003D\u003D nil, len(s) \u003D\u003D 0)\u000A');codeLines.push(' s \u003D make([]string, 3)\u000A fmt.Println(\"emp:\", s, \"len:\", len(s), \"cap:\", cap(s))\u000A');codeLines.push(' s[0] \u003D \"a\"\u000A s[1] \u003D \"b\"\u000A s[2] \u003D \"c\"\u000A fmt.Println(\"set:\", s)\u000A fmt.Println(\"get:\", s[2])\u000A');codeLines.push(' fmt.Println(\"len:\", len(s))\u000A');codeLines.push(' s \u003D append(s, \"d\")\u000A s \u003D append(s, \"e\", \"f\")\u000A fmt.Println(\"apd:\", s)\u000A');codeLines.push(' c :\u003D make([]string, len(s))\u000A copy(c, s)\u000A fmt.Println(\"cpy:\", c)\u000A');codeLines.push(' l :\u003D s[2:5]\u000A fmt.Println(\"sl1:\", l)\u000A');codeLines.push(' l \u003D s[:5]\u000A fmt.Println(\"sl2:\", l)\u000A');codeLines.push(' l \u003D s[2:]\u000A fmt.Println(\"sl3:\", l)\u000A');codeLines.push(' t :\u003D []string{\"g\", \"h\", \"i\"}\u000A fmt.Println(\"dcl:\", t)\u000A');codeLines.push(' t2 :\u003D []string{\"g\", \"h\", \"i\"}\u000A if slices.Equal(t, t2) {\u000A fmt.Println(\"t \u003D\u003D t2\")\u000A }\u000A');codeLines.push(' twoD :\u003D make([][]int, 3)\u000A for i :\u003D 0; i \u003C 3; i++ {\u000A innerLen :\u003D i + 1\u000A twoD[i] \u003D make([]int, innerLen)\u000A for j :\u003D 0; j \u003C innerLen; j++ {\u000A twoD[i][j] \u003D i + j\u000A }\u000A }\u000A fmt.Println(\"2d: \", twoD)\u000A}\u000A');codeLines.push('');codeLines.push('');codeLines.push('');
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