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

You were almost there! rle will work very nicely, you just need to sort your table on ID before computing rle:

CT <- data.frame( value = runif(10) , id = sample(5,10,repl=T) )

#  sort on ID when calculating rle
Count <- rle( sort( CT$id ) )

#  match values
CT$Count <- Count[[1]][ match( CT$id , Count[[2]] ) ]
CT
#       value id Count
#1  0.94282600  1     4
#2  0.12170165  2     2
#3  0.04143461  1     4
#4  0.76334609  3     2
#5  0.87320740  4     1
#6  0.89766749  1     4
#7  0.16539820  1     4
#8  0.98521044  5     1
#9  0.70609853  3     2
#10 0.75134208  2     2