title: Rbind in variable row size not giving NA's
body: <p>The initial data frame <code>mergedDf</code> is </p>
  
  <pre><code>  PROD_CODE
    1 PRD0900033,PRD0900135,PRD0900220,PRD0900709
    2                       PRD0900097,PRD0900550
    3                                  PRD0900121
    4                                  PRD0900353
    5                       PRD0900547,PRD0900614
  </code></pre>
  
  <p>After calling</p>
  
  <pre><code>mergedDf&lt;-data.frame(do.call('rbind', strsplit(as.character(mergedDf$PROD_CODE),',',fixed=TRUE)))
  </code></pre>
  
  <p>Output becomes</p>
  
  <pre><code> X1         X2         X3         X4
  1 PRD0900033 PRD0900135 PRD0900220 PRD0900709
  2 PRD0900097 PRD0900550 PRD0900097 PRD0900550
  3 PRD0900121 PRD0900121 PRD0900121 PRD0900121
  4 PRD0900353 PRD0900353 PRD0900353 PRD0900353
  5 PRD0900547 PRD0900614 PRD0900547 PRD0900614
  </code></pre>
  
  <p>It seems the excess rows are being refilled.</p>
  
  <p>I have tries using <code>bind_rows()</code>, <code>rbind_all()</code> but these require changing the splitted ones to data.frame which I couldn't do. I also tried using <code>rbindlist()</code> which also needed a data.frame as an argument.</p>
  
  <p>I need the ouput to be. The positions do not matter.</p>
  
  <pre><code>  X1         X2         X3         X4
  1 PRD0900033 PRD0900135 PRD0900220 PRD0900709
  2 PRD0900097 PRD0900550 NA         NA
  3 PRD0900121 NA         NA         NA
  4 PRD0900353 NA         NA         NA
  5 PRD0900547 PRD0900614 NA         NA
  </code></pre>
  
  <p>Or if anyone could recommend a nicer way to format for apriori algorithm implementation it would be nice.
  Please Help.</p>