Create a sequential number (counter) for rows within each group of a dataframe [duplicate]

Answer 1

Assuming your data are in a data.frame named Data, this will do the trick:

# ensure Data is in the correct order
Data <- Data[order(Data$personid),]
# tabulate() calculates the number of each personid
# sequence() creates a n-length vector for each element in the input,
# and concatenates the result
Data$id <- sequence(tabulate(Data$personid))