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copied!<p>Use <a href="http://www.ruby-doc.org/core-2.1.3/Kernel.html#method-i-rand" rel="noreferrer"><code>rand(range)</code></a></p> <p>From <a href="http://www.codeodor.com/index.cfm/2007/3/25/Ruby-random-numbers/1042" rel="noreferrer">Ruby Random Numbers</a>:</p> <blockquote> <p>If you needed a random integer to simulate a roll of a six-sided die, you'd use: <code>1 + rand(6)</code>. A roll in craps could be simulated with <code>2 + rand(6) + rand(6)</code>.</p> <p>Finally, if you just need a random float, just call <code>rand</code> with no arguments.</p> </blockquote> <hr> <p>As <a href="https://stackoverflow.com/users/8279/marc-andre-lafortune">Marc-André Lafortune</a> mentions in <a href="https://stackoverflow.com/questions/198460/how-to-get-a-random-number-in-ruby/2773866#2773866">his answer below (go upvote it)</a>, <a href="http://www.ruby-lang.org/en/news/2009/07/20/ruby-1-9-2-preview-1-released/" rel="noreferrer">Ruby 1.9.2 has its own <code>Random</code> class</a> (that Marc-André himself <a href="http://redmine.ruby-lang.org/issues/show/3104" rel="noreferrer">helped to debug</a>, hence the <a href="http://redmine.ruby-lang.org/versions/show/11" rel="noreferrer">1.9.2 target</a> for that feature).</p> <p>For instance, in this <a href="http://www.eggheadcafe.com/software/aspnet/35817496/random-integer-within-a-r.aspx" rel="noreferrer">game where you need to guess 10 numbers</a>, you can initialize them with:</p> <pre><code>10.times.map{ 20 + Random.rand(11) } #=> [26, 26, 22, 20, 30, 26, 23, 23, 25, 22] </code></pre> <p>Note: </p> <ul> <li><p>Using <code>Random.new.rand(20..30)</code> (using <code>Random.new</code>) generally would not be a good idea, as explained in detail (again) by <a href="https://stackoverflow.com/users/8279/marc-andre-lafortune">Marc-André Lafortune</a>, in <a href="https://stackoverflow.com/questions/198460/how-to-get-a-random-number-in-ruby/2773866#2773866">his answer</a> (again).</p></li> <li><p>But if you don't use <code>Random.new</code>, then the <a href="http://www.ruby-doc.org/core/classes/Random.src/M000693.html" rel="noreferrer">class method <code>rand</code></a> only takes a <code>max</code> value, not a <code>Range</code>, as <a href="https://stackoverflow.com/users/66725/banister">banister</a> (energetically) points out in the comment (and as documented in the <a href="http://www.ruby-doc.org/core/classes/Random.html" rel="noreferrer">docs for <code>Random</code></a>). Only the <a href="http://www.ruby-doc.org/core/classes/Random.src/M000688.html" rel="noreferrer">instance method</a> can take a <code>Range</code>, as illustrated by <a href="https://stackoverflow.com/questions/5722881/generate-a-random-number-with-7-digits/5723105#5723105">generate a random number with 7 digits</a>.</p></li> </ul> <p>This is why the equivalent of <code>Random.new.rand(20..30)</code> would be <code>20 + Random.rand(11)</code>, since <code>Random.rand(int)</code> returns “a random integer greater than or equal to zero and <em>less than the argument</em>.” <code>20..30</code> includes 30, I need to come up with a random number between 0 and 11, excluding 11.</p>

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