[R] Creating a new by variable in a dataframe

[ramoss]

Hello,

I have a dataframe w/ 3 variables of interest: transaction,date(tdate) &
time(event_tim).
How could I create a 4th variable (last_trans) that would flag the last
transaction of the day for each day?
In SAS I use:
proc sort data=all6;
by tdate event_tim;
run;
         /*Create last transaction flag per day*/
data all6;
  set all6;
  by tdate event_tim;
  last_trans=last.tdate;

Thanks ahead for any suggestions.



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