title: regroup rows by value in a table and return a counter of the line collapsed
body: <p>i'm stuck with this little problem:</p>
  
  <p><a href="https://i.stack.imgur.com/Wreaj.png" rel="nofollow noreferrer">df sample</a></p>
  
  <p>i want to collapse the column UnstableDependenciesPackage and i know i can do it with the function group_by</p>
  
  <pre><code>UD %&gt;% group_by(UnstableDependenciesPackage, version, project, Type)
  </code></pre>
  
  <p>the problem is that i want to summarize those occurences in the NumBadDep column, so i'll get for example:</p>
  
  <pre><code>org.apache.tools.ant.util - 1
  
  org.apache.tools.ant.taskdefs - 3
  
  org.apache.tools.ant.types - 2
  </code></pre>
  
  <p>and so on. But i miss the second step of my instruction</p>