title: Counting all unique values in a data set
body: <p>I have a data set of 60 plus computers with each column being the computer and the rows being the collection of all the software installed from each PC. I want to be able to count each unique value(software), so I can see how many of each software is currently installed. </p>
  
  <pre><code>data = [['a','a','c'],['a','b','d'],['a','c','c']]
  df = pd.DataFrame(data,columns=['col1','col2','col3'])
  df
  
  
  col1  col2  col3
  a      a      c
  a      b      d
  a      c      c
  
  I expect the following output
  a 4
  b 1
  c 3
  </code></pre>