# Returning the nth largest values of a vector in R

Suppose I have a vector in R of the following six values:
`U <- c(5, 12, 23, -7, 12, 3)`,
and I want the two largest values (or their indices in the vector). In this simple example it’s clear that the largest values are 23 and 12, and their indices are either 3 and 2, or 3 and 5 (depending on how we define the problem in the case of tied values).

One solution to this is:

`sort(U, decreasing=T)[1:2]`

to get the actual values, and

`order(U, decreasing=T)[1:2]`

to get the indices (which in this case is 3 and 2).

However, it is well known that sorting is typically an $O(n log(n))$ problem, whereas selecting the $k$th largest or smallest value is a linear $O(kn)$ problem.

No problems, partial sort can solve this problem. Partial sorting partitions the vector into two with the first $k$ being the smallest values of the vector:

`sort(U, partial=2)`

gives

`[1] -7 3 12 5 23 12`

however, partial sorting in R can’t sort in reverse order for some reason! But of course, we can sort `-U` instead. So:

`(-sort(-U, partial=2))[1:2]`

solves the problem of quickly returning the 2 largest values of a vector.

But finding the indices of these values is somewhat harder (although it probably shouldn’t be!). Looking at the help for the functions `order` and `sort.list`, we see that neither of these support partial sorting. Another option would be to `cbind` the vector `U` to `1:6` and partially sort the table as before by the first column, but as far as I can tell this is not possible.

So here is the solution I came up with:

```argsortN <- function(a_vector, N){
Nth.largest <- -sort(-a_vector, partial=N)[N] # from before

# Find the indices of all values greater than the nth largest:
ix.gte.Nth.largest <- which(a_vector >= Nth.largest)

# The next line sorts these in decreasing order:
argN <- ix.gte.Nth.largest[order(a_vector[ix.gte.Nth.largest], decreasing=T)]

# The only problem is if there are ties, the length of argN can be greater than N:
return(argN[1:N])
}
```

The exact indices returned by this function in the case of ties will depend on the way sorting of ties is handled by `order`.