how to assign a unique identifier to multiple data frame entries

Question

I have a large data frame that has three identifiers. For example:

df <- data.frame(year=c(1999,1999,2000,2000,2000), country=c('K','K','M','M','S'), 
                 site=c('di','se','di','di','di'))

Which will produce a data frame like this:

    year country site
    1999    K     di
    1999    K     se
    2000    M     di
    2000    M     di
    2000    S     di

I want to add an additional column to the data frame and have a 'unique id' assigned by using the entries for 'year', 'country', and 'site'. It would look something like this:

    year country site unique_id
    1999    K     di     1
    1999    K     se     2
    2000    M     di     3
    2000    M     di     3
    2000    S     di     4

Any suggestions on how to do this would be greatly appreciated. I'm thinking it could somehow be done using the plyr package?

Answer 1

This should work quite nicely. (It takes advantage of the fact that unique levels of a factor are each actually stored as integers, and uses as.numeric() to access/extract those integer values).

df$unique_id <- 
    as.numeric(as.factor(with(df, paste(year, country, site, sep="_"))))
df
#   year country site unique_id
# 1 1999       K   di         1
# 2 1999       K   se         2
# 3 2000       M   di         3
# 4 2000       M   di         3
# 5 2000       S   di         4