title: Select every nth row, offset the start and repeat
body: <p>I am trying to create a new column in a data.frame that is created by selecting the 9th row of a column starting at the first row (i.e. row 1, row 9, row 17). Once it reaches the nth row of the column I need it to repeat this process starting at row 2 (selecting row 2, row 10, row 18). I have a fixed number of rows at 96 so I need it to repeat until it would start on the 9th row and then quit.</p>
  
  <p>Here is an example of what I would like to do:</p>
  
  <pre><code>df &lt;- data.frame(Row=1:96)
  &gt; df$nineth &lt;- c(1,9,17,25,33,41,49,57,65,73,81,89,2,10,18,26,34,42,50,58,66,74,82,90)
  &gt; print(df)
     Row nineth
  1    1      1
  2    2      9
  3    3     17
  4    4     25
  5    5     33
  6    6     41
  7    7     49
  8    8     57
  9    9     65
  10  10     73
  11  11     81
  12  12     89
  13  13      2
  14  14     10
  15  15     18
  16  16     26
  17  17     34
  18  18     42
  19  19     50
  20  20     58
  21  21     66
  22  22     74
  23  23     82
  24  24     90
  </code></pre>
  
  <p>Is there a way to do this using a <code>for</code> loop? I am more familiar with them than the <code>apply</code> family.</p>