CODEWITHSUNDEEP
pythonlist
qin wang
qin wang

Reputation: 1

Strange results when swapping items in Python list

When I try to swap 2 items in a list, some strange things happen. When trying this code, the nums is updated to [4, 1, 3, 1]:

i = 1
nums = [-1, 4, 3, 1]
nums[i]
nums[nums[i]-1] = nums[nums[i]-1]
nums[i]

However, when the command is

nums[nums[i]-1]
nums[i] = nums[i]
nums[nums[i]-1]

The nums is [-1, 1, 3, 4], which is what I expect.

I am totally confused about what happens in the two cases. Could anyone explain this clearly?

Upvotes: 0

Views: 52

Answers (1)

PIG208
PIG208

Reputation: 2380

What you're trying to do is called multiple assignment in Python. Consider the following codes:

[print(0)][0], [print(1)][0] = print(2), print(3)

Outputs:

2
3
0
1

[print(0)][0] = [print(1)][0] = [print(2)][0] = [print(3)][0]

Outputs:

3
0
1
2

You might have found that for either of these assignments, the right-hand operands are evaluated first from left to right, and then it goes with left-hand operands.

It works the same way in the code you provided.

i = 1
nums = [-1, 4, 3, 1]
nums[i], nums[nums[i]-1] = nums[nums[i]-1], nums[i]

In the right-hand side, nums[nums[i]-1] is evaluated to be nums[3], which gives integer 1, and then nums[i] is evaluated to be nums[1], which gives integer 4.

Now it's the turn to assign values for the left-hand operands. At the moment, nums[i] refers to nums[1], thus 1 is assigned to nums[1]. Note that nums[1] equals 1 now, and then the interpreter finds that nums[nums[i]-1] refers to nums[1-1] which is nums[0]! Then 4 is assigned to nums[0], giving you the result [4, 1, 3 ,1].

However, if you switch the operands, the order of this multiple assignment will change, and the result will be different. Similarily, the evaluation process is as follows:

nums: [-1, 4, 3, 1]
nums[nums[i]-1], nums[i] = nums[i], nums[nums[i]-1]

nums: [-1, 4, 3, 1]
nums[nums[i]-1], nums[i] = 4, nums[nums[i]-1]

nums: [-1, 4, 3, 1]
nums[nums[i]-1], nums[i] = 4, 1

nums: [-1, 4, 3, 1]
nums[3], nums[i] = 4, 1

nums: [-1, 4, 3, 4]
nums[i] = 1

nums: [-1, 1, 3, 4]

Upvotes: 1

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