Nth smallest number in an unsorted array

nthsmallest.rb

def nthsmallest arr,startpos,endpos,nth 

  def choose_pivot arr,startpos,endpos

    #THis is for selecting median of three , you can return a random number as well. 

    if ((arr[startpos] < arr[endpos]) and (arr[startpos] > arr[startpos+ (endpos-startpos)/2])) or ((arr[startpos] > arr[endpos]) and (arr[startpos] < arr[startpos+ (endpos-startpos)/2]))      
      return startpos
    elsif ((arr[endpos] < arr[startpos]) and (arr[endpos] > arr[startpos+ (endpos-startpos)/2])) or ((arr[endpos] > arr[startpos]) and (arr[endpos] < arr[startpos+ (endpos-startpos)/2]))
      return endpos
    else
      return startpos+ (endpos-startpos)/2
    end

  end

  if (startpos - endpos == 0) and (nth == startpos)
    return arr[startpos]
  end
    
  pivot = choose_pivot(arr,startpos,endpos)
  arr[startpos],arr[pivot] = arr[pivot],arr[startpos]
  part_pos = startpos + 1
  for i in startpos+1..endpos
    if arr[startpos] > arr[i]
      arr[part_pos],arr[i] = arr[i],arr[part_pos]
      part_pos += 1
    end
  end

  arr[startpos],arr[part_pos-1] = arr[part_pos-1],arr[startpos]

  if nth < (part_pos-1)
    ret = nthsmallest(arr,startpos,part_pos-2,nth) if (part_pos - startpos - 2) >= 0
  elsif nth > (part_pos-1)
    ret = nthsmallest(arr,part_pos,endpos,nth) if endpos-part_pos >= 0
  else
    ret = arr[part_pos-1]
  end

  return ret 
  
end