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rlistduplicates

List of identical maximum column values in R

I have a matrix similar to below.

mat <- matrix(c(3,2,7,8,5,12,4,5,8,9,1,12,4,17,1,2,6,17,8,5),byrow=T,nrow=4)
colnames(mat) <- c(1,2,3,4,5); rownames(mat) <- c(1,2,3,4)
mat
   1  2  3  4 5
1  3  2  7  8 5
2 12  4  5  8 9
3  1 12  4 17 1
4  2  6 17  8 5

I am wanting to get the maximum of each column which I can get using:

apply(mat,2,max)

 1  2  3  4  5 
12 12 17 17  9

But I would like to have the duplicate indexes in a list like:

[[1]]
[1] 1 2

[[2]]
[1] 3 4

[[3]]
[1] 5

How would I go about doing so?

Solution

  • Try

    > unname(split(1:ncol(mat), Reduce(pmax, asplit(mat, 1))))
[[1]]
[1] 5

[[2]]
[1] 1 2

[[3]]
[1] 3 4

    or

    > aggregate(ind ~ ., stack(Reduce(pmax, asplit(mat, 1))), as.numeric)[["ind"]]
[[1]]
[1] 5

[[2]]
[1] 1 2

[[3]]
[1] 3 4