title: R Sum columns by index
body: <p><p>I need to find a way to sum columns by their index,I'm working on a big<code>read.csv</code> file, I'll show here a sample of the problem; I'd like for example to sum from the 2nd to the 5th and from the 6th to the 7h the following matrix:</p></p>
  
  <pre><code>a 1  3  3  4  5  6 
  b 2  1  4  3  4  1 
  c 1  3  2  1  1  5 
  d 2  2  4  3  1  3 
  </code></pre>
  
  <p>The result has to be like this:</p>
  
  <pre><code>a 11 11
  b 10  5
  c  7  6
  d  8  4
  </code></pre>
  
  <p>The columns have all different names</p>