counting occurrences in column and create variable in R

Question

I am new on R and I have a data.frame , called \"CT\", containing a column called \"ID\" containing several hundreds of different identification numbers (these are patients)

Answer 1

data.table usually provides the quickest way

set.seed(3)
library(data.table)
ct <- data.table(id=sample(1:10,15,replace=TRUE),item=round(rnorm(15),3))
st <- ct[,countid:=.N,by=id]
 id   item countid
 1:  2  0.953       2
 2:  9  0.535       2
 3:  4 -0.584       2
 4:  4 -2.161       2
 5:  7 -1.320       3
 6:  7  0.810       3
 7:  2  1.342       2
 8:  3  0.693       1
 9:  6 -0.323       5
10:  7 -0.117       3
11:  6 -0.423       5
12:  6 -0.835       5
13:  6 -0.815       5
14:  6  0.794       5
15:  9  0.178       2