R: Create sorted numeric IDs in a datafile with alphanumeric IDs

gistfile1.txt

# we simulate a data set with an alphanumeric ID column and some
# observed values x
# The goal is to give out  new participant IDs with consecutive numbers
# starting from 1 (while maintaining the original order)
dat <- data.frame(id=c("P1","P1","P1","second","second","second","3rd","no4","no4"),
           x=rnorm(9))

# convert IDs to a factor and then use position of factor level as new ID
# /!\ this yields ID that are according to alphanumeric sorting of original IDs
dat$numeric_id <- as.numeric(factor(dat$id))

print(dat)

# if you want IDs to match the original order, explicitly state
# that the levels correspond to the original ordering of unique
# values
dat$numeric_id <- as.numeric(factor(dat$id,levels = unique(dat$id)))

print(dat)