How to identify coordinates in R

Question

I would like to identify which coordinate of my vector gives me the greatest value. For a simple example suppose that:

x <- c(10,22,20,18,5)

Answer 1

which.max is your friend as pointed out by @Hong Ooi

> x <- c(10,22,20,18,5)
> which.max(x)
[1] 2

Another (not optimal way) is a combination of which and max.

> which(x==max(x))
[1] 2

Answer 2

Because you say co-ordinates, I am assuming the case-in-point may not always be a one-dimensional vector and therefore I am going to make my comment to @Jilber an answer.

A general answer is to use which(x == max(x), ind.arr = TRUE). This will give you all dimensions of an array of any dimensionality. For e.g.

R> x <- array(runif(8), dim=rep_len(2, 3))                                                                                                                              
R> x
, , 1

          [,1]      [,2]
[1,] 0.3202624 0.7740697
[2,] 0.9374742 0.2370483

, , 2

          [,1]      [,2]
[1,] 0.9423731 0.2099402
[2,] 0.7035772 0.8195685

R> which(x == max(x), arr.ind=TRUE)
     dim1 dim2 dim3
[1,]    1    1    2

R> which(x[1, , ] == max(x[1, , ]), arr.ind=TRUE)
     row col
[1,]   1   2

R> which(x[1, 1, ] == max(x[1, 1, ]), arr.ind=TRUE)
[1] 2

For the specific case of one-dimensional vectors, which.max is a 'faster' solution.

Answer 3

First, find the greatest value with max:

> max(x)
[1] 22

From there, you can figure out which value(s) in the vector match the greatest value:

> x==max(x)
[1] FALSE  TRUE FALSE FALSE FALSE

which() can be used to translate the boolean vector into indices:

which(x==max(x))
[1] 2