title: How to subtract 1 from each value in a column in Pandas
body: <p>I think this should be a simple problem, but I can't find a solution. </p>
  
  <p>Within a subset of rows in a dataframe, I need to decrement the value of each item in a column by 1. 
  I have tried various approaches, but the values continue to be unchanged. 
  Following another entry on SO, I tried </p>
  
  <pre><code>def minus1(x):
      x =x-1
      return x
  
  pledges[pledges.Source == 'M0607'].DayOFDrive = pledges[pledges.Source == 'M0607'].DayOFDrive.map(minus1)
  </code></pre>
  
  <p>When I typed </p>
  
  <pre><code>pledges[pledges.Source == 'M0607'].DayOFDrive
  </code></pre>
  
  <p>to check it, the original unchanged data came back. 
  I have also tried </p>
  
  <pre><code>pledges[pledges.Source == 'M0607'].DayOFDrive = pledges[pledges.Source == 'M0607'].DayOFDrive-1
  </code></pre>
  
  <p>which also does nothing. </p>
  
  <p>How can I reduce all the values in a column by 1 for a subset of rows ?</p>