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<title>Go by Example: Random Numbers</title>
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<div class="example" id="random-numbers">
<h2><a href="./">Go by Example</a>: Random Numbers</h2>
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<p>Go&rsquo;s <code>math/rand</code> package provides
<a href="http://en.wikipedia.org/wiki/Pseudorandom_number_generator">pseudorandom number</a>
generation.</p>
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<a href="http://play.golang.org/p/ZdFpbahgC1"><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;time&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;math/rand&quot;</span>
</pre></div>
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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>
</pre></div>
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<p>For example, <code>rand.Intn</code> returns a random <code>int</code> n,
<code>0 &lt;= n &lt; 100</code>.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">),</span> <span class="s">&quot;,&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">rand</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">))</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">()</span>
</pre></div>
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<p><code>rand.Float64</code> returns a <code>float64</code> <code>f</code>,
<code>0.0 &lt;= f &lt; 1.0</code>.</p>
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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="nx">rand</span><span class="p">.</span><span class="nx">Float64</span><span class="p">())</span>
</pre></div>
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<p>This can be used to generate random floats in
other ranges, for example <code>5.0 &lt;= f' &lt; 10.0</code>.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">((</span><span class="nx">rand</span><span class="p">.</span><span class="nx">Float64</span><span class="p">()</span><span class="o">*</span><span class="mi">5</span><span class="p">)</span><span class="o">+</span><span class="mi">5</span><span class="p">,</span> <span class="s">&quot;,&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">((</span><span class="nx">rand</span><span class="p">.</span><span class="nx">Float64</span><span class="p">()</span> <span class="o">*</span> <span class="mi">5</span><span class="p">)</span> <span class="o">+</span> <span class="mi">5</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">()</span>
</pre></div>
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<p>The default number generator is deterministic, so it&rsquo;ll
produce the same sequence of numbers each time by default.
To produce varying sequences, give it a seed that changes.
Note that this is not safe to use for random numbers you
intend to be secret, use <code>crypto/rand</code> for those.</p>
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<div class="highlight"><pre> <span class="nx">s1</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">NewSource</span><span class="p">(</span><span class="nx">time</span><span class="p">.</span><span class="nx">Now</span><span class="p">().</span><span class="nx">UnixNano</span><span class="p">())</span>
<span class="nx">r1</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">New</span><span class="p">(</span><span class="nx">s1</span><span class="p">)</span>
</pre></div>
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<p>Call the resulting <code>rand.Rand</code> just like the
functions on the <code>rand</code> package.</p>
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<div class="highlight"><pre> <span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r1</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">),</span> <span class="s">&quot;,&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r1</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">))</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">()</span>
</pre></div>
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<p>If you seed a source with the same number, it
produces the same sequence of random numbers.</p>
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<div class="highlight"><pre> <span class="nx">s2</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">NewSource</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="nx">r2</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">New</span><span class="p">(</span><span class="nx">s2</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r2</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">),</span> <span class="s">&quot;,&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r2</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</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">s3</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">NewSource</span><span class="p">(</span><span class="mi">42</span><span class="p">)</span>
<span class="nx">r3</span> <span class="o">:=</span> <span class="nx">rand</span><span class="p">.</span><span class="nx">New</span><span class="p">(</span><span class="nx">s3</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r3</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">),</span> <span class="s">&quot;,&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Print</span><span class="p">(</span><span class="nx">r3</span><span class="p">.</span><span class="nx">Intn</span><span class="p">(</span><span class="mi">100</span><span class="p">))</span>
<span class="p">}</span>
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<div class="highlight"><pre><span class="gp">$</span> go run random-numbers.go
<span class="go">81,87</span>
<span class="go">0.6645600532184904</span>
<span class="go">7.123187485356329,8.434115364335547</span>
<span class="go">0,28</span>
<span class="go">5,87</span>
<span class="go">5,87</span>
</pre></div>
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<p>See the <a href="http://golang.org/pkg/math/rand/"><code>math/rand</code></a>
package docs for references on other random quantities
that Go can provide.</p>
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<p class="next">
Next example: <a href="number-parsing">Number Parsing</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/random-numbers">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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