title: Extracting rows from a matrix conditional on another variable
body: <p>How do I extract the rows of <code>m</code> where <code>a</code> is even? For example, </p>
  
  <pre><code>m = matrix(1:24, 6)
  a = c(3, 4, 1, 1, 2, 5)
  </code></pre>