title: R; separate columns with comma into rows
body: <p>I'm having trouble to split column into multiple rows even after using <code>separate_rows</code> function.
  It gives me the following error..</p>
  <blockquote>
  <p>Error: Can't subset columns that don't exist.</p>
  </blockquote>
  <p>INPUT:</p>
  <pre><code>ID      Colours         Shapes
  1       Red             Triangle  
  1       Red             Square  
  2       Green, Black    Circle
  2       Green, Black    Triangle  
  3       Blue            Square  
  3       Blue            Oval
  </code></pre>
  <p>OUTPUT:</p>
  <pre><code>ID      Colours         Shapes
  1       Red             Triangle  
  1       Red             Square  
  2       Green           Circle
  2       Green           Triangle
  2       Black           Circle
  2       Black           Triangle  
  3       Blue            Square  
  3       Blue            Oval
  </code></pre>