title: Finding the number of common elements between two arrays
body: <p>I need a way to find how many common elements in between two arrays.</p>
  
  <p>Suppose that I have:</p>
  
  <pre><code>a = {1,2,3,4,5,6,7,8,9}
  b = {2,4,6,8,10} 
  </code></pre>
  
  <p>So the result would be 4.</p>
  
  <p>I need it to be effective with very large array like arrays of length 1300000. I have tried the intersect method but it gave me a wrong number such as 256 or 11 and tried with method Zip but it's too slow.</p>