count distinct levels of a data frame for groups based on a condition

Question

I have the following DF

x = data.frame('grp' = c(1,1,1,2,2,2),'a' = c(1,2,1,1,2,1), 'b'= c(6,5,6,6,2,6), 'c' = c(0.1,0.2,0.4,-1, 0.9,0.7))

  grp a b    c
1   1 1 6  0.1
2   1 2 5  0.2
3   1 1 6  0.4
4   2 1 6 -1.0
5   2 2 2  0.9
6   2 1 6  0.7 

I want to count distinct levels of (a,b) for each group where c >= 0.1

I have tried using dplyr for this using group_by & summarise but not getting the desired result

x %>% group_by(grp) %>% summarise(count = n_distinct(c(a,b)[c >= 0.1]))

For the above case I would expect the following result

    grp count
  <dbl> <int>
1     1     2
2     2     2

However using the above query I am getting the following result

    grp count
  <dbl> <int>
1     1     4
2     2     3

Logically the above output seems to be solving for all unique values of a concat list of (a,b) but not what I require Any pointers, really appreciate any help

Answer 1

Here's another way using dplyr. It sounds like you want to filter based on c, so we do that. Instead of using c(a, b) in n_distinct, we can write it as n_distinct(a, b).

x %>%
    filter(c >= 0.1) %>%
    group_by(grp) %>%
    summarise(cnt_d = n_distinct(a, b))

#     grp cnt_d
#   <dbl> <int>
# 1     1     2
# 2     2     2

Answer 2

We can paste a and b columns and count distinct values in each group.

library(dplyr)

x %>% 
  mutate(col = paste(a, b, sep = "_")) %>%
  group_by(grp) %>%
  summarise(count = n_distinct(col[c >= 0.1]))

#    grp count
#  <dbl> <int>
#1     1     2
#2     2     2

Answer 3

An option using data.table

library(data.table)
setDT(x)[c >= 0.1, .(cnt_d = uniqueN(paste(a, b))), .(grp)]
#    grp cnt_d
#1:   1     2
#2:   2     2