Swapping in python is not giving correct output when done by using a variable

Question

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

gives unexpected and wrong output 4,1,3,1 whereas,

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

gives expected correct output -1,1,3,4

Can anyone justify why using a variable k in swapping is not leading to the correct output ?

To start with , maybe expression arr[arr[k]-1] has the new value of arr[k] in index arr[k]-1 ? — Anurag Kedia
– Anurag Kedia, Commented Dec 26, 2021 at 14:52
Explain exactly what you want to do. For any k, arr[k]-1 isn't guaranteed to be in the list, so that's not a safe index to use for swapping. For example, if arr = [1, 9, 3, 5] — OneCricketeer
– OneCricketeer, Commented Dec 26, 2021 at 14:53
It's related to the evaluation order Multiple assignment and evaluation order in Python. arr[arr[k]-1] uses the newly assigned arr[k] value. — shriakhilc
– shriakhilc, Commented Dec 26, 2021 at 14:55

Himanshu Jangid · Accepted Answer · 2021-12-26 14:58:13Z

1

I think it's because the 4 at index 1 gets swapped at some point. so when we swap 1 and for for the first time is might be okey.. but then they are already swapped.. so the value of k-1 gets changed.

answered Dec 26, 2021 at 14:58

Himanshu Jangid

3854 silver badges15 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Swapping in python is not giving correct output when done by using a variable

1 Answer 1

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related