title: Add rows for missing data in a sequence
body: <p>I have a data frame with three columns give below: </p>
  
  <ol>
  <li>A catalog code</li>
  <li>Spend on that catalog code in a given week</li>
  <li>The number of the week in the period</li>
  </ol>
  
  <p>Some dummy data is below.</p>
  
  <pre><code>cat.code&lt;-c("a","a","a","b","b","b","c","d")
  dol&lt;-c(4,10,2,5,6,8,9,10)
  wk.purch&lt;-c(1,2,3,1,5,2,3,4)
  
  fk.data&lt;-data.frame(cat.code,dol,wk.purch)
  &gt; fk.data
    cat.code dol wk.purch
  1        a   4        1
  2        a  10        2
  3        a   2        3
  4        b   5        1
  5        b   6        5
  6        b   8        2
  7        c   9        3
  8        d  10        4
  </code></pre>
  
  <p>I want to add rows that have <code>dol = 0</code> when there is no purchase in a given week.</p>
  
  <p>So, looking at <code>cat.code= a</code>, I would want to transform this data frame to add two more rows for weeks 4 and 5 that have 0 for <code>dol</code>. I have tried using <code>expand.grid()</code> and <code>merge</code> together, but that isn't working for me. </p>
  
  <p>Thank you, all! </p>