Count unique values of a column by pairwise combinations of another column in R

Question

Let\'s say I have the following data frame:

   ID Code
1   1    A
2   1    B
3   1    C
4   2    B
5   2    C
6   2    D
7   3    C
8   3    A
9   3    D
10          


        

Answer 1

Here is a data.table way to solve the problem. Use combn function to pick up all possible combinations of Code and then count ID for each unique CodeComb:

library(data.table)
setDT(df)[, .(CodeComb = sapply(combn(Code, 2, simplify = F), 
                                function(cmb) paste(sort(cmb), collapse = ", "))), .(ID)]
# list all combinations of Code for each ID
         [, .(IdCount = .N), .(CodeComb)]    
# count number of unique id for each code combination

#    CodeComb IdCount
# 1:     A, B       2
# 2:     A, C       2
# 3:     B, C       3
# 4:     B, D       3
# 5:     C, D       2
# 6:     A, D       1