Combinations by group in R -

i have question combinations group.

my mini-sample looks this:

sample <- data.frame(   group=c("a","a","a","a","b","b","b"),   number=c(1,2,3,2,4,5,3) ) 

if apply function of combnto data frame,it gives me following result, combinations of values under 'number' column regardless of group value belongs to:

         [,1] [,2]    [1,]    1    2    [2,]    1    3    [3,]    1    2    [4,]    1    4    [5,]    1    5    [6,]    1    3    [7,]    2    3    [8,]    2    2    [9,]    2    4   [10,]    2    5   [11,]    2    3   [12,]    3    2   [13,]    3    4   [14,]    3    5   [15,]    3    3   [16,]    2    4   [17,]    2    5   [18,]    2    3   [19,]    4    5   [20,]    4    3   [21,]    5    3 

the code used results above follows:

t(combn((sample$number), 2)) 

however, combination results within group (i.e., "a", "b"). therefore, result want should this:

     [,1] [,2] [,3] [1,]      1    2 [2,]      1    3 [3,]      1    2 [4,]      2    3 [5,]      2    2 [6,]      3    2 [7,]   b    4    5 [8,]   b    4    3 [9,]   b    5    3 

in addition combinations, column indicating group.

thank in advance. looking forward replies!

we can use group function data.table

library(data.table) setdt(sample)[, {i1 <-  combn(number, 2)                    list(i1[1,], i1[2,]) }, =  group] #    group v1 v2 #1:      1  2 #2:      1  3 #3:      1  2 #4:      2  3 #5:      2  2 #6:      3  2 #7:     b  4  5 #8:     b  4  3 #9:     b  5  3 

or compact option be

setdt(sample)[, transpose(combn(number, 2, fun = list)), = group] 

---

or using base r

 lst <- by(sample$number, sample$group, fun = combn, m= 2)  data.frame(group = rep(unique(as.character(sample$group)),                          sapply(lst, ncol)), t(do.call(cbind, lst)))