title: Mean of Multiple Columns in R
body: <p>I am trying take the mean of a list of columns in R and am running into a issue.  Let's say I have:</p>
  
  <pre><code>A B  C  D
  1 2  3  4
  5 6  7  8
  9 10 11 12
  </code></pre>
  
  <p>What I am trying to do is take the mean of columns c(A,C) and save it as a value say (E) as well as the mean of columns c(B,D) and have it save as a different value say F. Is that possible?</p>
  
  <pre><code>E   F
  2   3
  6   7
  10  11
  </code></pre>