Loop to return number from string given conditions R

I have a question. I have a column indicating the district as a string “district1”, for example. However, now I would like to ass another column with just number of the district. I am using if else statements but this is exhausting.

Below I give just a example of the more than 30 districts I have:

### generate dataframe
 d <- data.frame(variable=c("district1", "district1", "district1", "district1", "district2", "district2", "district3"))


 d$id1 = ifelse(d$variable=="district1", 1,
                        ifelse(d$variable=="district2", 2,
                               ifelse(d$variable=="district3", 3, NA)))

Thus I was wondering if it was possible to do it with a loop. However the looping structure is not well done:

#  for (i in 1:30){
#    d$id1 = ifelse(d$variable=="districti", i, NA) 
# }

Could someone help me to do in a more elegant way? This would be great Best