ruby quicksort error - ruby

Question

below is my quicksort implementation in ruby(using the first element as pivot). But there's an error I cannot figure out why
def quicksort(items)
return items if items.nil? or items.length <= 1
first=items[0]
left,right=parti(items,0,items.length)
quicksort(left) + [first] + quicksort(right)
end
def parti(s,l,r)
p=s[l]
i=l+1
(l+1..r).each do |x|
if s[x] < p
s[x],s[i] = s[i],s[x]
i+=1
end
end
s[l],s[i-1] = s[i-1],s[l]
return [s[0..i-2],s[i..r]]
end
The error:
putarray.rb:38:in `block in parti': undefined method `<' for nil:NilClass (NoM
hodError)
from inputarray.rb:37:in `each'
from inputarray.rb:37:in `parti'
from inputarray.rb:22:in `quicksort'
from inputarray.rb:47:in `<main>'
it says in
if s[x] < p
s[x] is NilClass.
UPDATE: It turns out
left,right=parti(items,0,items.length) should be
left,right=parti(items,0,items.length-1)
But after this change, an error
inputarray.rb:37: stack level too deep (SystemStackError)
point to
(l+1..r).each do |x|
Didn't find any good explanation on the internet.

Answer 1

Keep in mind that in Ruby, array index starts from 0, not 1.
Therefore items.length will return the value of 1 greater than the maximum index in the array.
Try right=parti(items,0,items.length-1) instead of right=parti(items,0,items.length).