How to number/label data-table by group-number from group_by?

Updated answer

get_group_number = function(){
    i = 0
    function(){
        i <<- i+1
        i
    }
}
group_number = get_group_number()
df %>% group_by(u,v) %>% mutate(label = group_number())

You can also consider the following slightly unreadable version

group_number = (function(){i = 0; function() i <<- i+1 })()
df %>% group_by(u,v) %>% mutate(label = group_number())

---

using iterators package

library(iterators)

counter = icount()
df %>% group_by(u,v) %>% mutate(label = nextElem(counter))

Another approach using data.table would be

require(data.table)
setDT(df)[,label:=.GRP, by = c("u", "v")]

which results in:

    u v label
 1: 2 1     1
 2: 1 3     2
 3: 2 1     1
 4: 3 4     3
 5: 3 1     4
 6: 1 1     5
 7: 3 2     6
 8: 2 3     7
 9: 3 2     6
10: 3 4     3

As of dplyr version 1.0.4, the function cur_group_id() has replaced the older function group_indices.

Call it on the grouped data.frame:

df %>%
  group_by(u, v) %>%
  mutate(label = cur_group_id())

# A tibble: 10 x 3
# Groups:   u, v [6]
       u     v label
   <int> <int> <int>
 1     2     2     4
 2     2     2     4
 3     1     3     2
 4     3     2     6
 5     1     4     3
 6     1     2     1
 7     2     2     4
 8     2     4     5
 9     3     2     6
10     2     4     5

dplyr has a group_indices() function that you can use like this:

df %>% 
    mutate(label = group_indices(., u, v)) %>% 
    group_by(label) ...