title: Change Value of a Dataframe Column Based on a Filter
body: <p>I have a Dataframe that consists of 2 columns: </p>
  
  <ol>
  <li>"Time Spent on website"</li>
  <li>"Dollars spent on the website"</li>
  </ol>
  
  <p>I want to perform some classification analysis on this dataset and I only care whether a user made a purchase or not.  So I want to run through the "Dollars spent on the website" column and transform the value to "1" if the user spent over $0.00 and have the value be "0" if the user spent nothing.</p>
  
  <p>What is the proper way to do this with a pandas dataframe?</p>