title: How to select rows based on pre-determined gaps (R)?
body: <p>I recently post another question asking how I could create a new data.frame based on a colunm variable. I thought it would fix my problem but I realize now that I was asking the wrong thing.
  What I mean with my question is, how I can select rows in a constant gap and create a new data.frame with them?
  Like, if I have:</p>
  
  <pre><code>1   A   B   C
  2   D   E   F
  3   G   H   I
  4   J   K   L
  5   M   N   O
  6   P   Q   R 
  </code></pre>
  
  <p>I will want to select the rows that grow in two to two like:</p>
  
  <pre><code>2  D   E   F
  4  J   K   L
  6  P   Q   R
  </code></pre>
  
  <p>But actually in my case, I need to select the rows that are groing in 40 to 40 and create a new data.frame with them.</p>
  
  <p>Sorry for another post, but I will be really glad if you guys could help me. I'm a new user of R.</p>