Assign unique ID based on two columns [duplicate]

Answer 1

We can do this in base R without doing any group by operation

df$ID <- cumsum(!duplicated(df[1:2]))
df
#   School Student Year ID
#1      A      10 1999  1
#2      A      10 2000  1
#3      A      20 1999  2
#4      A      20 2000  2
#5      A      20 2001  2
#6      B      10 1999  3
#7      B      10 2000  3

NOTE: Assuming that 'School' and 'Student' are ordered

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Or using tidyverse

library(dplyr)
df %>% 
    mutate(ID = group_indices_(df, .dots=c("School", "Student"))) 
#  School Student Year ID
#1      A      10 1999  1
#2      A      10 2000  1
#3      A      20 1999  2
#4      A      20 2000  2
#5      A      20 2001  2
#6      B      10 1999  3
#7      B      10 2000  3

As @radek mentioned, in the recent version (dplyr_0.8.0), we get the notification that group_indices_ is deprecated, instead use group_indices

df %>% 
   mutate(ID = group_indices(., School, Student))