public/string-formatting

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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="docs">
            
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          <td class="code leading">
            <a href="https://go.dev/play/p/EZCZX1Uwp6D"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <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 class="docs">
            
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          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="p">(</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;fmt&#34;</span>
</span></span><span class="line"><span class="cl">    <span class="s">&#34;os&#34;</span>
</span></span><span class="line"><span class="cl"><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            
          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">point</span> <span class="kd">struct</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="nx">x</span><span class="p">,</span> <span class="nx">y</span> <span class="kt">int</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            
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          <td class="code leading">
            
          <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 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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <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></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;struct1: %v\n&#34;</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>If the value is a struct, the <code>%+v</code> variant will
include the struct&rsquo;s field names.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;struct2: %+v\n&#34;</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span></span></span></code></pre>
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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>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;struct3: %#v\n&#34;</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>To print the type of a value, use <code>%T</code>.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;type: %T\n&#34;</span><span class="p">,</span> <span class="nx">p</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>Formatting booleans is straight-forward.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;bool: %t\n&#34;</span><span class="p">,</span> <span class="kc">true</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>There are many options for formatting integers.
Use <code>%d</code> for standard, base-10 formatting.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;int: %d\n&#34;</span><span class="p">,</span> <span class="mi">123</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>This prints a binary representation.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;bin: %b\n&#34;</span><span class="p">,</span> <span class="mi">14</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>This prints the character corresponding to the
given integer.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;char: %c\n&#34;</span><span class="p">,</span> <span class="mi">33</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p><code>%x</code> provides hex encoding.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;hex: %x\n&#34;</span><span class="p">,</span> <span class="mi">456</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>There are also several formatting options for
floats. For basic decimal formatting use <code>%f</code>.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;float1: %f\n&#34;</span><span class="p">,</span> <span class="mf">78.9</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p><code>%e</code> and <code>%E</code> format the float in (slightly
different versions of) scientific notation.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;float2: %e\n&#34;</span><span class="p">,</span> <span class="mf">123400000.0</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;float3: %E\n&#34;</span><span class="p">,</span> <span class="mf">123400000.0</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>For basic string printing use <code>%s</code>.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;str1: %s\n&#34;</span><span class="p">,</span> <span class="s">&#34;\&#34;string\&#34;&#34;</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>To double-quote strings as in Go source, use <code>%q</code>.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;str2: %q\n&#34;</span><span class="p">,</span> <span class="s">&#34;\&#34;string\&#34;&#34;</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <p>As with integers 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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;str3: %x\n&#34;</span><span class="p">,</span> <span class="s">&#34;hex this&#34;</span><span class="p">)</span></span></span></code></pre>
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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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;pointer: %p\n&#34;</span><span class="p">,</span> <span class="o">&amp;</span><span class="nx">p</span><span class="p">)</span></span></span></code></pre>
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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>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;width1: |%6d|%6d|\n&#34;</span><span class="p">,</span> <span class="mi">12</span><span class="p">,</span> <span class="mi">345</span><span class="p">)</span></span></span></code></pre>
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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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;width2: |%6.2f|%6.2f|\n&#34;</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></span></span></code></pre>
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          <td class="docs">
            <p>To left-justify, use the <code>-</code> flag.</p>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;width3: |%-6.2f|%-6.2f|\n&#34;</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></span></span></code></pre>
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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>

          </td>
          <td class="code leading">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;width4: |%6s|%6s|\n&#34;</span><span class="p">,</span> <span class="s">&#34;foo&#34;</span><span class="p">,</span> <span class="s">&#34;b&#34;</span><span class="p">)</span></span></span></code></pre>
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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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">&#34;width5: |%-6s|%-6s|\n&#34;</span><span class="p">,</span> <span class="s">&#34;foo&#34;</span><span class="p">,</span> <span class="s">&#34;b&#34;</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <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">&#34;sprintf: a %s&#34;</span><span class="p">,</span> <span class="s">&#34;string&#34;</span><span class="p">)</span>
</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="nx">s</span><span class="p">)</span></span></span></code></pre>
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          <td class="docs">
            <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">
            
          <pre class="chroma"><code><span class="line"><span class="cl">    <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">&#34;io: an %s\n&#34;</span><span class="p">,</span> <span class="s">&#34;error&#34;</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            
          </td>
          <td class="code">
            
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run string-formatting.go
</span></span><span class="line"><span class="cl"><span class="go">struct1: {1 2}
</span></span></span><span class="line"><span class="cl"><span class="go">struct2: {x:1 y:2}
</span></span></span><span class="line"><span class="cl"><span class="go">struct3: main.point{x:1, y:2}
</span></span></span><span class="line"><span class="cl"><span class="go">type: main.point
</span></span></span><span class="line"><span class="cl"><span class="go">bool: true
</span></span></span><span class="line"><span class="cl"><span class="go">int: 123
</span></span></span><span class="line"><span class="cl"><span class="go">bin: 1110
</span></span></span><span class="line"><span class="cl"><span class="go">char: !
</span></span></span><span class="line"><span class="cl"><span class="go">hex: 1c8
</span></span></span><span class="line"><span class="cl"><span class="go">float1: 78.900000
</span></span></span><span class="line"><span class="cl"><span class="go">float2: 1.234000e+08
</span></span></span><span class="line"><span class="cl"><span class="go">float3: 1.234000E+08
</span></span></span><span class="line"><span class="cl"><span class="go">str1: &#34;string&#34;
</span></span></span><span class="line"><span class="cl"><span class="go">str2: &#34;\&#34;string\&#34;&#34;
</span></span></span><span class="line"><span class="cl"><span class="go">str3: 6865782074686973
</span></span></span><span class="line"><span class="cl"><span class="go">pointer: 0xc0000ba000
</span></span></span><span class="line"><span class="cl"><span class="go">width1: |    12|   345|
</span></span></span><span class="line"><span class="cl"><span class="go">width2: |  1.20|  3.45|
</span></span></span><span class="line"><span class="cl"><span class="go">width3: |1.20  |3.45  |
</span></span></span><span class="line"><span class="cl"><span class="go">width4: |   foo|     b|
</span></span></span><span class="line"><span class="cl"><span class="go">width5: |foo   |b     |
</span></span></span><span class="line"><span class="cl"><span class="go">sprintf: a string
</span></span></span><span class="line"><span class="cl"><span class="go">io: an error</span></span></span></code></pre>
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