R: All possible pairs of products present in one transaction

Question

I have data of such structure:

data<-data.frame(TransactionNumber=rep(1:10, each=4), 
                 Product=rep(c("a","b","c","d","e","f","g","c","i","j"), length.out=40))

  TransactionNumber Product
1                  1       a
2                  1       b
3                  1       c
4                  1       d
5                  2       e
6                  2       f
7                  2       g
8                  2       c
9                  3       i
10                 3       j
11                 3       a
12                 3       b
13                 4       c
14                 4       d
15                 4       e
16                 4       f
17                 5       g
18                 5       c
19                 5       i
20                 5       j
21                 6       a
22                 6       b
23                 6       c
24                 6       d
25                 7       e
26                 7       f
27                 7       g
28                 7       c
29                 8       i
30                 8       j
31                 8       a
32                 8       b
33                 9       c
34                 9       d
35                 9       e
36                 9       f
37                10       g
38                10       c
39                10       i
40                10       j

I'm trying to get for each transaction numbers all possible combinations of two products present in given transaction (without repetitions). Each row would be one combination. So for transaction one I would get 1.ab 2. ac etc. I was unsuccessfully struggling with this for a while. Any ideas of simple solution? Thank you!

Answer 1

require(plyr)
ddply(data, "TransactionNumber", .fun=function(x) {
  t(combn(as.character(x$Product), 2))
})
#    TransactionNumber 1 2
# 1                  1 a b
# 2                  1 a c
# 3                  1 a d
# 4                  1 b c
# 5                  1 b d
# 6                  1 c d
# 7                  2 e f
# 8                  2 e g
# 9                  2 e c
# 10                 2 f g
# 11                 2 f c
# 12                 2 g c
# 13                 3 i j
# ...