How to make sequences that may be empty? - r

Question

I recently tried to adapt some pseudocode for an in-place quicksort, quoted below:
function quicksort(array)
if length(array) > 1
pivot := select any element of array
left := first index of array
right := last index of array
while left ≤ right
while array[left] < pivot
left := left + 1
while array[right] > pivot
right := right - 1
if left ≤ right
swap array[left] with array[right]
left := left + 1
right := right - 1
quicksort(array from first index to right)
quicksort(array from left to last index)
Following this, I wrote this code:
quicksort<-function(array)
{
len<-length(array)
if(len>1)
{
left<-1
right<-len
pivot<-array[(left+right)%/%2]
while(left<=right)
{
while(array[left]<pivot){left<-left+1}
while(array[right]<pivot){right<-right-1}
if(left<=right)
{
array[c(left,right)]<-array[c(right,left)]
left<-left+1
right<-right-1
}
}
array<-quicksort(array[1:right])#Bug here
array<-quicksort(array[left:len])
}
array
}
If you run this code with more than one integer as an input, you'll find that it eventually tries to sort lists of NAs. I suspect that the problem is that the pseduocode wants array from first index to right - my 1:right - to be read as an empty sequence when right is zero. As is well known, R would actually read as 1:0 as the sequence 0 1.
Is there any function to get the behavior that I intend? I could do this with an if statement, but R is usually good enough with sequences that I can't help but think that there will be a better way. I tried to use seq, but it will throw an error if you try to use anything like seq(from=1,to=0,by=1).

Answer 1

There are two issues with the code:
One of the inequality signs is flipped.
The array assignment needs to be modified to only change a subset of elements rather than replace (and shrink) the whole array.
Here is the corrected code
quicksort <- function(array) {
len <- length(array)
if(len > 1) {
left <- 1
right <- len
pivot <- array[(left+right)%/%2]
while(left <= right)
{
while(array[left] < pivot){
left <- left + 1
}
while(array[right] > pivot) { # Changed "<" to ">"
right <- right - 1
}
if(left <= right) {
array[c(left, right)] <- array[c(right, left)]
left <- left + 1
right <- right - 1
}
}
# Modified the following two lines to only set a subset of array
array[1:right] <- quicksort(array[1:right])
array[left:len] <- quicksort(array[left:len])
}
array
}
quicksort(c(2, 6, 3, 1, 4, 5))
#> [1] 1 2 3 4 5 6