R delete rows in data frame where nrow of index is smaller than certain value

Question

I want to delete certain rows in a data frame when the number of rows with the same index is smaller than a pre-specified value.

> fof.6.5[1:15, 1:3]
   draw          


        

Answer 1

Here is another option using data.table. Convert the 'data.frame' to 'data.table' (setDT(df1), grouped by 'draw', if the nrows (.N) are greater than or equal to 'n' then get the Subset of Data.table (.SD)

library(data.table)
n <- 5
setDT(df1)[, if(.N >= n) .SD, by = draw]  
#    draw Fund.ID Firm.ID
# 1:    1    1667     666
# 2:    1    1572     622
# 3:    1    1392     553
# 4:    1     248      80
# 5:    1    3223     332
# 6:    2    2959    1998
# 7:    2    2659    1561
# 8:    2   14233    2517
# 9:    2   10521   12579
#10:    2    3742    1045

If we want only 'n' number of rows, use the head

setDT(df1)[, if(.N >= n) head(.SD, n), by = draw]