Assign a serial number to duplicated values of a vector in R

Question

I would like to assign a serial number to duplicated values of a vector in R without using a for loop as shown in the example below. I have a big data set. Thanks

a<-c("A","B","A","A","B","C","D","E","D")

Expected Output:

desired.answer<-c(1,1,2,3,2,1,1,1,2)

question from:https://stackoverflow.com/questions/65944871/assign-a-serial-number-to-duplicated-values-of-a-vector-in-r

Answer 1

Using the data you provided you can use the group_by function from the dplyr package to then assign serial numbers independently to each group.

data.frame(value = c("A","B","A","A","B","C","D","E","D") ) %>% 
  group_by(value) %>% 
  mutate(serial_number = 1:n())

A tibble: 9 x 2
Groups:   value [5]
 value serial_number
 <chr>         <int>
1 A                 1
2 B                 1
3 A                 2
4 A                 3
5 B                 2
6 C                 1
7 D                 1
8 E                 1
9 D                 2