elements exchange

Question

I met this in game section:

arr = [4,3,1,2]
arr[0], arr[arr[0]-1] = arr[arr[0]-1], arr[0]
print(arr)

but coldn’t understand why correct answer is [2,4,1,2]
I guess this code changes or reassigns the 0-th and the 3-rd elements (4,2). How it connected with the 1-st element (3)?

Answer 1

The trick for understand this is valutate first the right side of assignment.
Following your example:

arr[0], arr[arr[0]-1] = arr[arr[0]-1],arr[0]

and apply the "trick":

arr[0], arr[arr[0]-1] = arr[4-1],4

arr[0], arr[arr[0]-1] = 2,4

Then assign:
arr[0]= 2
arr[arr[0]-1]= arr[2-1]=
arr[1]= 4

Get the resulting array [2,4,1,2]

Answer 2

KrOW thank’s. I understand my mistake: i thought that when assigne elements in one string it takes them in one moment (parallel). But it works consecutively like it was two separate strings with assignments. Right?

Answer 3

Anna Starikova Yes, nothing happen in "parallel" in assignment. The imortant thing to remember is than its valuted FIRST the right side then assigned sequentially to left side.