title: Subtract two columns in a data frame
body: <p>I have a data frame with several columns, I need to subtract the values of two columns from this data frame and add the resulted values as a new column in the same dataset.</p>
  
  <p>Any idea on how to write function in R to do so?</p>
  
  <p>Thanks in advance</p>
  
  <p>Far</p>