Create numerical discrete values if values in a column equal in R

I have a column of IDs in a dataframe that sometimes has duplicates, take for example,

What I want to do is take this column and create another column that indicates what ID the row belongs to. i.e. I want it to look like,

Essentially, I want to loop through the IDs and if it equals to a previous one, I indicate that it is from the same ID, and if it is a new ID, I create a new indicator for it.

Does anyone know is there a quick function in R that I could do this with? Or have any other suggestions?

>Solution :

Convert to factor with levels ordered with unique (order of appearance in the data set) and then to numeric:

data$IDCategory <- as.numeric(factor(data$ID, levels = unique(data$ID)))

#> data
#   ID IDCategory
#1 209          1
#2 315          2
#3 109          3
#4 315          2
#5 451          4
#6 209          1