Python Swap Issue

Question

Just wondering what happens when I run

nums[i], nums[nums[i] - 1] = nums[nums[i] - 1], nums[i] in python3.

I know it works when I do like x = nums[i] - 1 nums[i], nums[x] = nums[x], nums[i] But still don't know what exactly happened.

Answer 1

• To Swap elements correctly use this

nums=[4,8,3,7,5]
print('before',nums)
i=3
nums[i], nums[i - 1] = nums[i - 1], nums[i]
print('after',nums)

before [4, 8, 3, 7, 5]
after [4, 8, 7, 3, 5]

• To Know why your code doesn't work see the following code

the code will bec executed from the most right to left

nums=[4,8,7,3,5]
i=3

>nums[i], nums[nums[i] - 1] = nums[nums[i] - 1], nums[i]
>nums[i], nums[nums[i] - 1] = nums[3 - 1], 3
>nums[i], nums[nums[i] - 1] = nums[2], 3
>nums[i], nums[nums[i] - 1] = 7, 3

#first element
>nums[i] ----> 7
>nums[3] ----> 7
>nums=[4,8,7,7,5]

#second element
>nums[nums[i] - 1] ---> 3
>nums[nums[3] - 1] ---> 3 #nums[3] became 7 
>nums[7 - 1] ---> 3
>nums[6] ---> 3  #index out of range