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After go1.16, go will use module mode by default, even when the repository is checked out under GOPATH or in a one-off directory. Add go.mod, go.sum to keep this repo buildable without opting out of the module mode. > go mod init github.com/mmcgrana/gobyexample > go mod tidy > go mod vendor In module mode, the 'vendor' directory is special and its contents will be actively maintained by the go command. pygments aren't the dependency the go will know about, so it will delete the contents from vendor directory. Move it to `third_party` directory now. And, vendor the blackfriday package. Note: the tutorial contents are not affected by the change in go1.16 because all the examples in this tutorial ask users to run the go command with the explicit list of files to be compiled (e.g. `go run hello-world.go` or `go build command-line-arguments.go`). When the source list is provided, the go command does not have to compute the build list and whether it's running in GOPATH mode or module mode becomes irrelevant.
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201 lines
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<!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: Pointers</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 = 'recursion';
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}
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if (e.key == "ArrowRight") {
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window.location.href = 'structs';
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}
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}
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</script>
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<body>
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<div class="example" id="pointers">
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<h2><a href="./">Go by Example</a>: Pointers</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Go supports <em><a href="http://en.wikipedia.org/wiki/Pointer_(computer_programming)">pointers</a></em>,
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allowing you to pass references to values and records
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within your program.</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="http://play.golang.org/p/oimmXypnAcs"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"fmt"</span>
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</pre></div>
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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’ll show how pointers work in contrast to values with
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2 functions: <code>zeroval</code> and <code>zeroptr</code>. <code>zeroval</code> has an
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<code>int</code> parameter, so arguments will be passed to it by
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value. <code>zeroval</code> will get a copy of <code>ival</code> distinct
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from the one in the calling function.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">zeroval</span><span class="p">(</span><span class="nx">ival</span> <span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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<span class="nx">ival</span> <span class="p">=</span> <span class="mi">0</span>
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<span class="p">}</span>
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</pre></div>
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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>zeroptr</code> in contrast has an <code>*int</code> parameter, meaning
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that it takes an <code>int</code> pointer. The <code>*iptr</code> code in the
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function body then <em>dereferences</em> the pointer from its
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memory address to the current value at that address.
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Assigning a value to a dereferenced pointer changes the
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value at the referenced address.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">zeroptr</span><span class="p">(</span><span class="nx">iptr</span> <span class="o">*</span><span class="kt">int</span><span class="p">)</span> <span class="p">{</span>
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<span class="o">*</span><span class="nx">iptr</span> <span class="p">=</span> <span class="mi">0</span>
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<span class="p">}</span>
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</pre></div>
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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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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
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<span class="nx">i</span> <span class="o">:=</span> <span class="mi">1</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"initial:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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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<div class="highlight"><pre> <span class="nx">zeroval</span><span class="p">(</span><span class="nx">i</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"zeroval:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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>&i</code> syntax gives the memory address of <code>i</code>,
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i.e. a pointer to <code>i</code>.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">zeroptr</span><span class="p">(</span><span class="o">&</span><span class="nx">i</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"zeroptr:"</span><span class="p">,</span> <span class="nx">i</span><span class="p">)</span>
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</pre></div>
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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>Pointers can be printed too.</p>
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</td>
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<td class="code">
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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="s">"pointer:"</span><span class="p">,</span> <span class="o">&</span><span class="nx">i</span><span class="p">)</span>
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<span class="p">}</span>
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</pre></div>
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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><code>zeroval</code> doesn’t change the <code>i</code> in <code>main</code>, but
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<code>zeroptr</code> does because it has a reference to
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the memory address for that variable.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> go run pointers.go
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<span class="go">initial: 1</span>
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<span class="go">zeroval: 1</span>
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<span class="go">zeroptr: 0</span>
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<span class="go">pointer: 0x42131100</span>
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</pre></div>
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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="structs">Structs</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> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/pointers">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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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import \"fmt\"\u000A');codeLines.push('func zeroval(ival int) {\u000A ival \u003D 0\u000A}\u000A');codeLines.push('func zeroptr(iptr *int) {\u000A *iptr \u003D 0\u000A}\u000A');codeLines.push('func main() {\u000A i :\u003D 1\u000A fmt.Println(\"initial:\", i)\u000A');codeLines.push(' zeroval(i)\u000A fmt.Println(\"zeroval:\", i)\u000A');codeLines.push(' zeroptr(\u0026i)\u000A fmt.Println(\"zeroptr:\", i)\u000A');codeLines.push(' fmt.Println(\"pointer:\", \u0026i)\u000A}\u000A');codeLines.push('');
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</script>
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<script src="site.js" async></script>
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