为什么在执行外循环时数组的长度必须在它旁边有一个负数

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