SAS data step, example BY and RETAIN

data_step_by_retain.sas

data dataOut; 
set dataIn; 
  /* repeat for each firm */
  by gvkey; 
  /* remember totsale across observations */
  retain totsale; 
  /* set sum to zero for first obs for each firm */
  if first.gvkey then totsale = 0;
  /* add sale to total, you may need to set sale to 0 if missing
    e.g. if sale eq . then sale = 0;
  */
  totsale = totsale + sale; 
  /* output to dataset when last obs for each firm, omitting 
  this step will give all observations */
  if last.gvkey then output;
run;