title: Extract numeric elements from a vector
body: <p>I have a vector 'a' which has numeric and non-numeric elements. I want to extract the numeric elements from the vector and build a new vector. How do I do this?</p>
  
  <p>So if a is:</p>
  
  <pre><code>a = c(15, 3, 44, "hello", 4, "NA")
  </code></pre>
  
  <p>I want to get a vector with four elements namely elements 15 3 44 and 4</p>