461 lines
18 KiB
Plaintext
Generated
461 lines
18 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: String Formatting</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 = 'string-functions';
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}
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if (e.key == "ArrowRight") {
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window.location.href = 'regular-expressions';
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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="string-formatting">
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<h2><a href="./">Go by Example</a>: String Formatting</h2>
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<table>
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<tr>
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<td class="docs">
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<p>Go offers excellent support for string formatting in
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the <code>printf</code> tradition. Here are some examples of
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common string formatting tasks.</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/L6BkGeaN_p4"><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"><span class="kn">package</span> <span class="nx">main</span>
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</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"><span class="kn">import</span> <span class="p">(</span>
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<span class="s">"fmt"</span>
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<span class="s">"os"</span>
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<span class="p">)</span>
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</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"><span class="kd">type</span> <span class="nx">point</span> <span class="kd">struct</span> <span class="p">{</span>
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<span class="nx">x</span><span class="p">,</span> <span class="nx">y</span> <span class="kt">int</span>
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<span class="p">}</span>
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</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"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
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</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>Go offers several printing “verbs” designed to
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format general Go values. For example, this prints
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an instance of our <code>point</code> struct.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">p</span> <span class="o">:=</span> <span class="nx">point</span><span class="p">{</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">}</span>
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<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="nx">p</span><span class="p">)</span>
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</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>If the value is a struct, the <code>%+v</code> variant will
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include the struct’s field names.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<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="nx">p</span><span class="p">)</span>
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</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>%#v</code> variant prints a Go syntax representation
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of the value, i.e. the source code snippet that
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would produce that value.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<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="nx">p</span><span class="p">)</span>
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</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 print the type of a value, use <code>%T</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%T\n"</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span>
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</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>Formatting booleans is straight-forward.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%t\n"</span><span class="p">,</span> <span class="kc">true</span><span class="p">)</span>
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</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 are many options for formatting integers.
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Use <code>%d</code> for standard, base-10 formatting.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%d\n"</span><span class="p">,</span> <span class="mi">123</span><span class="p">)</span>
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</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 prints a binary representation.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%b\n"</span><span class="p">,</span> <span class="mi">14</span><span class="p">)</span>
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</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 prints the character corresponding to the
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given integer.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%c\n"</span><span class="p">,</span> <span class="mi">33</span><span class="p">)</span>
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</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>%x</code> provides hex encoding.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%x\n"</span><span class="p">,</span> <span class="mi">456</span><span class="p">)</span>
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</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 are also several formatting options for
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floats. For basic decimal formatting use <code>%f</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%f\n"</span><span class="p">,</span> <span class="mf">78.9</span><span class="p">)</span>
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</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>%e</code> and <code>%E</code> format the float in (slightly
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different versions of) scientific notation.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%e\n"</span><span class="p">,</span> <span class="mf">123400000.0</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%E\n"</span><span class="p">,</span> <span class="mf">123400000.0</span><span class="p">)</span>
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</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>For basic string printing use <code>%s</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%s\n"</span><span class="p">,</span> <span class="s">"\"string\""</span><span class="p">)</span>
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</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 double-quote strings as in Go source, use <code>%q</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%q\n"</span><span class="p">,</span> <span class="s">"\"string\""</span><span class="p">)</span>
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</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>As with integers seen earlier, <code>%x</code> renders
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the string in base-16, with two output characters
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per byte of input.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%x\n"</span><span class="p">,</span> <span class="s">"hex this"</span><span class="p">)</span>
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</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 print a representation of a pointer, use <code>%p</code>.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%p\n"</span><span class="p">,</span> <span class="o">&</span><span class="nx">p</span><span class="p">)</span>
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</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>When formatting numbers you will often want to
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control the width and precision of the resulting
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figure. To specify the width of an integer, use a
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number after the <code>%</code> in the verb. By default the
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result will be right-justified and padded with
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spaces.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"|%6d|%6d|\n"</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">345</span><span class="p">)</span>
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</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 specify the width of printed floats,
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though usually you’ll also want to restrict the
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decimal precision at the same time with the
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width.precision syntax.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"|%6.2f|%6.2f|\n"</span><span class="p">,</span> <span class="mf">1.2</span><span class="p">,</span> <span class="mf">3.45</span><span class="p">)</span>
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</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 left-justify, use the <code>-</code> flag.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"|%-6.2f|%-6.2f|\n"</span><span class="p">,</span> <span class="mf">1.2</span><span class="p">,</span> <span class="mf">3.45</span><span class="p">)</span>
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</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 may also want to control width when formatting
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strings, especially to ensure that they align in
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table-like output. For basic right-justified width.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"|%6s|%6s|\n"</span><span class="p">,</span> <span class="s">"foo"</span><span class="p">,</span> <span class="s">"b"</span><span class="p">)</span>
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</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 left-justify use the <code>-</code> flag as with numbers.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"|%-6s|%-6s|\n"</span><span class="p">,</span> <span class="s">"foo"</span><span class="p">,</span> <span class="s">"b"</span><span class="p">)</span>
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</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>So far we’ve seen <code>Printf</code>, which prints the
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formatted string to <code>os.Stdout</code>. <code>Sprintf</code> formats
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and returns a string without printing it anywhere.</p>
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</td>
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<td class="code leading">
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<pre class="chroma">
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<span class="nx">s</span> <span class="o">:=</span> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Sprintf</span><span class="p">(</span><span class="s">"a %s"</span><span class="p">,</span> <span class="s">"string"</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
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</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 format+print to <code>io.Writers</code> other than
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<code>os.Stdout</code> using <code>Fprintf</code>.</p>
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</td>
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<td class="code">
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<pre class="chroma">
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<span class="nx">fmt</span><span class="p">.</span><span class="nf">Fprintf</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">"an %s\n"</span><span class="p">,</span> <span class="s">"error"</span><span class="p">)</span>
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<span class="p">}</span>
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</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">
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<pre class="chroma"><span class="gp">$</span> go run string-formatting.go
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<span class="go">{1 2}
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</span><span class="go">{x:1 y:2}
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</span><span class="go">main.point{x:1, y:2}
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</span><span class="go">main.point
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</span><span class="go">true
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</span><span class="go">123
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</span><span class="go">1110
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</span><span class="go">!
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</span><span class="go">1c8
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</span><span class="go">78.900000
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</span><span class="go">1.234000e+08
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</span><span class="go">1.234000E+08
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</span><span class="go">"string"
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</span><span class="go">"\"string\""
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</span><span class="go">6865782074686973
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</span><span class="go">0x42135100
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</span><span class="go">| 12| 345|
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</span><span class="go">| 1.20| 3.45|
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</span><span class="go">|1.20 |3.45 |
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</span><span class="go">| foo| b|
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</span><span class="go">|foo |b |
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</span><span class="go">a string
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</span><span class="go">an error</span></pre>
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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="regular-expressions">Regular Expressions</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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<script>
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var codeLines = [];
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codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"fmt\"\u000A \"os\"\u000A)\u000A');codeLines.push('type point struct {\u000A x, y int\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' p :\u003D point{1, 2}\u000A fmt.Printf(\"%v\\n\", p)\u000A');codeLines.push(' fmt.Printf(\"%+v\\n\", p)\u000A');codeLines.push(' fmt.Printf(\"%#v\\n\", p)\u000A');codeLines.push(' fmt.Printf(\"%T\\n\", p)\u000A');codeLines.push(' fmt.Printf(\"%t\\n\", true)\u000A');codeLines.push(' fmt.Printf(\"%d\\n\", 123)\u000A');codeLines.push(' fmt.Printf(\"%b\\n\", 14)\u000A');codeLines.push(' fmt.Printf(\"%c\\n\", 33)\u000A');codeLines.push(' fmt.Printf(\"%x\\n\", 456)\u000A');codeLines.push(' fmt.Printf(\"%f\\n\", 78.9)\u000A');codeLines.push(' fmt.Printf(\"%e\\n\", 123400000.0)\u000A fmt.Printf(\"%E\\n\", 123400000.0)\u000A');codeLines.push(' fmt.Printf(\"%s\\n\", \"\\\"string\\\"\")\u000A');codeLines.push(' fmt.Printf(\"%q\\n\", \"\\\"string\\\"\")\u000A');codeLines.push(' fmt.Printf(\"%x\\n\", \"hex this\")\u000A');codeLines.push(' fmt.Printf(\"%p\\n\", \u0026p)\u000A');codeLines.push(' fmt.Printf(\"|%6d|%6d|\\n\", 12, 345)\u000A');codeLines.push(' fmt.Printf(\"|%6.2f|%6.2f|\\n\", 1.2, 3.45)\u000A');codeLines.push(' fmt.Printf(\"|%-6.2f|%-6.2f|\\n\", 1.2, 3.45)\u000A');codeLines.push(' fmt.Printf(\"|%6s|%6s|\\n\", \"foo\", \"b\")\u000A');codeLines.push(' fmt.Printf(\"|%-6s|%-6s|\\n\", \"foo\", \"b\")\u000A');codeLines.push(' s :\u003D fmt.Sprintf(\"a %s\", \"string\")\u000A fmt.Println(s)\u000A');codeLines.push(' fmt.Fprintf(os.Stderr, \"an %s\\n\", \"error\")\u000A}\u000A');codeLines.push('');
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