Suppose we have a matrix M
M <- matrix(c(1:9),3,3)
diag(M) <- NA
M
[,1] [,2] [,3]
[1,] NA 4 7
[2,] 2 NA 8
[3,] 3 6 NA
where each entry describes the outcomes of pairwise interactions. Each interaction of row i with column j is interepreted as "object i outperformed object j X times". Examples: Object 2 performs better than object 1 in 2 cases. Object 1 performs better than object 3 in 7 cases.
Is there a quick way to transform this matrix into an object holding this information in a format where each row fully describes the interactions between two objects? The goal is something like this:
[,1] [,2] [,3] [,4]
[1,] "OBJ1" "OBJ2" "N1" "N2"
[2,] "1" "2" "4" "2"
[3,] "1" "3" "7" "3"
[4,] "2" "3" "8" "6"
where the first two columns give the objects that are compared while columns 3 and 4 describe how often OBJ1 outperformed OBJ2 and vice versa. The interpretation of the first row is: Object 1 has outperformed Object 2 4 times, whereas Object 2 has outperformed Object 1 2 times. I have been playing around with reshape2 and aggregating without useful results so far.
Maybe you can try the code below
inds <- t(combn(dim(M)[1], 2))
Mout <- `colnames<-`(
cbind(inds, M[inds], M[inds[, 2:1]]),
do.call(paste0, rev(expand.grid(1:2, c("Obj", "N"))))
)
which gives
> Mout
Obj1 Obj2 N1 N2
[1,] 1 2 4 2
[2,] 1 3 7 3
[3,] 2 3 8 6