title: Combination of two arrays in Ruby
body: <p>What is the Ruby way to achieve following?</p>
  
  <pre><code>a = [1,2]
  b = [3,4]
  </code></pre>
  
  <p>I want an array: </p>
  
  <pre><code>=&gt; [f(1,3) ,f(1,4) , f(2,3) ,f(2,4)]
  </code></pre>