Finding row index containing maximum value using R

Question

Given the following matrix lets assume I want to find the maximum value in column two:

mat <- matrix(c(1:3,7:9,4:6), byrow = T, nc = 3)

mat

     [,1] [,2] [,3]

[1,]    1    2    3

[2,]    7    8    9

[3,]    4    5    6

I know max(mat[,2]) will return 8. How can I return the row index, in this case, row two?

Answer 1

To find the index of the maximum element, you can use the which.max() function from the base package.

The basic syntax of which.max() is given below:

which.max(x)

Arguments:

x

numeric (logical, integer or double) vector or an R object for which the internal coercion to double works whose min or max is searched for.

In your case:

mat <- matrix(c(1:3,7:9,4:6), byrow = T, nc = 3)

which.max( mat[,2] )

[1] 2