Why does the length of an array have to have a negative next to it when doing the outer loop

Question

def sort(nums):
  for i in range(len(nums)-1,0,-1):
    for j in range(i):
      if nums[j]>nums[j+1]:
        temp = nums[j]
        nums[j] = nums[j+1]
        nums[j+1] = temp

nums = [5,3,8,6,7,2] sort(nums) print(nums)

I am a beginner in python and i am learning bubble sort my question is what is the effect of putting -1 next to len(nums).

Answer 1

The range takes three arguments, where the first argument is the first number in the sequence.

The first index you iterate over is the last element in the list, hence you use len(nums)-1 so you get the valid index of the last item in nums list, as indexes are zero-based in Python.

eg

nums       = [5,3,8,6,7,2] 
length     = len(nums)  # 6
last_index = length - 1 # 5