title: Select rows of a matrix that meet a condition
body: <p>In R with a matrix:</p>
  
  <pre><code>     one two three four
   [1,]   1   6    11   16
   [2,]   2   7    12   17
   [3,]   3   8    11   18
   [4,]   4   9    11   19
   [5,]   5  10    15   20
  </code></pre>
  
  <p>I want to extract the submatrix whose rows have column three = 11. That is:</p>
  
  <pre><code>      one two three four
   [1,]   1   6    11   16
   [3,]   3   8    11   18
   [4,]   4   9    11   19
  </code></pre>
  
  <p>I want to do this without looping. I am new to R so this is probably very obvious but the
  documentation is often somewhat terse.</p>