title: Adding CSV values together and updating the file with the sum?
body: <p>As an example in a CSV format, I have 20,3,1 printed. </p>
  
  <p>In this case there are:</p>
  
  <ul>
  <li>20 seats available</li>
  <li>3 reservations</li>
  <li>1 cancellation</li>
  </ul>
  
  <p>I need to subtract the second value from the first one, and then add the third value to that, making a total. Then outputted as: 20,3,1,18 The sum being 18. This is just an example as there will be different files with different numbers, yet in this same formatting, from different files all named the same.
  'The code so far is:</p>
  
  <pre><code>import csv
  
  with open ('transactions.csv') as csvfile:
      readCSV = csv.reader(csvfile, delimiter=',')
      print (readCSV)
  
  for row in readCSV:
      print(row)
  </code></pre>
  
  <p>The CSV files will contain multiple rows of this formatting:</p>
  
  <p>30,3,2</p>
  
  <p>20,4,1</p>
  
  <p>15,5,3</p>