title: Conditional copy of values from one column to another columns
body: <p>I have a pandas dataframe that looks something like this:</p>
  
  <pre><code> name        job             jobchange_rank   date
  Thisguy      Developer       1                2012
  Thisguy      Analyst         2                2014
  Thisguy      Data Scientist  3                2015
  Anotherguy   Developer       1                2018
  </code></pre>
  
  <p>The jobchange_rank represents the each individual's (based on name) ranked change in position, where rank nr 1 represent his/her first position nr 2 his/her second position, etc.</p>
  
  <p>Now for the fun part. I want to create a new column where I can see a person's previous job, something like this:</p>
  
  <pre><code> name        job             jobchange_rank   date   previous_job
  Thisguy      Developer       1                2012   None
  Thisguy      Analyst         2                2014   Developer
  Thisguy      Data Scientist  3                2015   Analyst
  Anotherguy   Developer       1                2018   None
  </code></pre>
  
  <p>I've created the following code to get the "None" values where there was no job change:</p>
  
  <pre><code>df.loc[df['jobchange_rank'].sub(df['jobchange_rank'].min()) == 0, 'previous_job'] = 'None'  
  </code></pre>
  
  <p>Sadly, I can't seem to figure out how to get the values from the other column where the needed condition applies. </p>
  
  <p>Any help is more then welcome!
  Thanks in advance.</p>