Applying a function to every combination of elements in a vector

Question

I would like to apply a certain (custom) function to all combinations of an array. I think its best to explain with an example:

Matrix 1 :

A B C
1          


        

Answer 1

This can be done simply if you install gtools and use the permutations function.

require(gtools)
M1 <- list(A=1, B=2, C=3)
M2 <- list(A=4, B=5, C=6)

perms <- t(permutations(3, 2, 1:3))

comboList <- list()
for (i in 1:ncol(perms)) {
    nameString <- paste0(names(M2)[perms[1,i]], names(M1)[perms[2,i]])
    comboList[[i]] <- mapply("/", M2[[perms[,i][1]]], M1[[perms[,i][2]]])
}

The mapply function is a pretty magical built in R function. It's worth while getting the know the entire family of *apply functions.

The output is in comboList, which is as follows:

> comboList
$AB
[1] 2

$AC
[1] 1.333333

$BA
[1] 5

$BC
[1] 1.666667

$CA
[1] 6

$CB
[1] 3