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gobyexample/public/string-formatting
Mark McGranaghan 6da8afaa7c Change Twitter link
2013-11-17 08:43:04 -08:00

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<title>Go by Example: String Formatting</title>
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<div class="example" id="string-formatting">
<h2><a href="./">Go by Example</a>: String Formatting</h2>
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<td class="docs">
<p>Go offers excellent support for string formatting in
the <code>printf</code> tradition. Here are some examples of
common string formatting tasks.</p>
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<td class="code empty leading">
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<a href="http://play.golang.org/p/qayRuqXHym"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;os&quot;</span>
</pre></div>
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<div class="highlight"><pre><span class="kd">type</span> <span class="nx">point</span> <span class="kd">struct</span> <span class="p">{</span>
<span class="nx">x</span><span class="p">,</span> <span class="nx">y</span> <span class="kt">int</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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<td class="docs">
<p>Go offers several printing &ldquo;verbs&rdquo; designed to
format general Go values. For example, this prints
an instance of our <code>point</code> struct.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <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>
<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="nx">p</span><span class="p">)</span>
</pre></div>
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<p>If the value is a struct, the <code>%+v</code> variant will
include the struct&rsquo;s field names.</p>
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<td class="code leading">
<div class="highlight"><pre> <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="nx">p</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>The <code>%#v</code> variant prints a Go syntax representation
of the value, i.e. the source code snippet that
would produce that value.</p>
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<td class="code leading">
<div class="highlight"><pre> <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="nx">p</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>To print the type of a value, use <code>%T</code>.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%T\n&quot;</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span>
</pre></div>
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<p>Formatting booleans is straight-forward.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%t\n&quot;</span><span class="p">,</span> <span class="kc">true</span><span class="p">)</span>
</pre></div>
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<p>There are many options for formatting integers.
Use <code>%d</code> for standard, base-10 formatting.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%d\n&quot;</span><span class="p">,</span> <span class="mi">123</span><span class="p">)</span>
</pre></div>
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<p>This prints a binary representation.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%b\n&quot;</span><span class="p">,</span> <span class="mi">14</span><span class="p">)</span>
</pre></div>
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<p>This prints the character corresponding to the
given integer.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%c\n&quot;</span><span class="p">,</span> <span class="mi">33</span><span class="p">)</span>
</pre></div>
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<p><code>%x</code> provides hex encoding.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%x\n&quot;</span><span class="p">,</span> <span class="mi">456</span><span class="p">)</span>
</pre></div>
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<p>There are also several formatting options for
floats. For basic decimal formatting use <code>%f</code>.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%f\n&quot;</span><span class="p">,</span> <span class="mf">78.9</span><span class="p">)</span>
</pre></div>
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<p><code>%e</code> and <code>%E</code> format the float in (slightly
different versions of) scientific notation.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%e\n&quot;</span><span class="p">,</span> <span class="mf">123400000.0</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;%E\n&quot;</span><span class="p">,</span> <span class="mf">123400000.0</span><span class="p">)</span>
</pre></div>
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<p>For basic string printing use <code>%s</code>.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%s\n&quot;</span><span class="p">,</span> <span class="s">&quot;\&quot;string\&quot;&quot;</span><span class="p">)</span>
</pre></div>
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<p>To double-quote strings as in Go source, use <code>%q</code>.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%q\n&quot;</span><span class="p">,</span> <span class="s">&quot;\&quot;string\&quot;&quot;</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>As with integers as seen earlier, <code>%x</code> renders
the string in base-16, with two output characters
per byte of input.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%x\n&quot;</span><span class="p">,</span> <span class="s">&quot;hex this&quot;</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>To print a representation of a pointer, use <code>%p</code>.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;%p\n&quot;</span><span class="p">,</span> <span class="o">&amp;</span><span class="nx">p</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>When formatting numbers you will often want to
control the width and precision of the resulting
figure. To specify the width of an integer, use a
number after the <code>%</code> in the verb. By default the
result will be right-justified and padded with
spaces.</p>
</td>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;|%6d|%6d|\n&quot;</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">345</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>You can also specify the width of printed floats,
though usually you&rsquo;ll also want to restrict the
decimal precision at the same time with the
width.precision syntax.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;|%6.2f|%6.2f|\n&quot;</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>
</pre></div>
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<p>To left-justify, use the <code>-</code> flag.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;|%-6.2f|%-6.2f|\n&quot;</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>
</pre></div>
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<td class="docs">
<p>You may also want to control width when formatting
strings, especially to ensure that they align in
table-like output. For basic right-justified width.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;|%6s|%6s|\n&quot;</span><span class="p">,</span> <span class="s">&quot;foo&quot;</span><span class="p">,</span> <span class="s">&quot;b&quot;</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>To left-justify use the <code>-</code> flag as with numbers.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">&quot;|%-6s|%-6s|\n&quot;</span><span class="p">,</span> <span class="s">&quot;foo&quot;</span><span class="p">,</span> <span class="s">&quot;b&quot;</span><span class="p">)</span>
</pre></div>
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<p>So far we&rsquo;ve seen <code>Printf</code>, which prints the
formatted string to <code>os.Stdout</code>. <code>Sprintf</code> formats
and returns a string without printing it anywhere.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">s</span> <span class="o">:=</span> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Sprintf</span><span class="p">(</span><span class="s">&quot;a %s&quot;</span><span class="p">,</span> <span class="s">&quot;string&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="nx">s</span><span class="p">)</span>
</pre></div>
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<p>You can format+print to <code>io.Writers</code> other than
<code>os.Stdout</code> using <code>Fprintf</code>.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">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">&quot;an %s\n&quot;</span><span class="p">,</span> <span class="s">&quot;error&quot;</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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</table>
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</td>
<td class="code">
<div class="highlight"><pre><span class="gp">$</span> go run string-formatting.go
<span class="go">{1 2}</span>
<span class="go">{x:1 y:2}</span>
<span class="go">main.point{x:1, y:2}</span>
<span class="go">main.point</span>
<span class="go">true</span>
<span class="go">123</span>
<span class="go">1110</span>
<span class="go">!</span>
<span class="go">1c8</span>
<span class="go">78.900000</span>
<span class="go">1.234000e+08</span>
<span class="go">1.234000E+08</span>
<span class="go">&quot;string&quot;</span>
<span class="go">&quot;\&quot;string\&quot;&quot;</span>
<span class="go">6865782074686973</span>
<span class="go">0x42135100</span>
<span class="go">| 12| 345|</span>
<span class="go">| 1.20| 3.45|</span>
<span class="go">|1.20 |3.45 |</span>
<span class="go">| foo| b|</span>
<span class="go">|foo |b |</span>
<span class="go">a string</span>
<span class="go">an error</span>
</pre></div>
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</table>
<p class="next">
Next example: <a href="regular-expressions">Regular Expressions</a>.
</p>
<p class="footer">
by <a href="https://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/string-formatting">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
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