I have a data frame "m" as shown below: I am trying to find each account's most active month (with most number of V1). for example for account "2", it will be "month 6", for account 3 it will be "month 1", .... I wrote the below loop, it works fine but just takes a long time even I only used 8000 rows, the whole data set has 250,000 rows, so the code below is not usable. Does any one can suggest a better way to achieve this? Thanks a lot.