Style/ParallelAssignment for array swapping

[ShockwaveNN]

I have code like this. I just need to swap two elements in array

array = [1, 2, 3, 4]
array[0], array[1] = array[1], array[0]
p array

rubocop game me this offence:

test.rb:2:1: C: Style/ParallelAssignment: Do not use parallel assignment.
array[0], array[1] = array[1], array[0]
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

By documentation of Style/ParallelAssignment cop I cannot understand that is wrong with my code.
Is this a bug in cop, or there is better way to swap elements in my array?

I use
rubocop 0.35.1 (using Parser 2.2.3.0, running on ruby 2.1.6 x86_64-linux)

[alexdowad]

Is this a bug in cop

Yes. It's a bug.

This cop tries to find an ordering of assignments which doesn't change the meaning of the code. If it can't find one, it's not supposed to issue a warning.

[kamen-hursev]

Just a clarification, Rubocop has the same bug for swapping values of hash or object attributes:
All these result in Style/ParallelAssignment offences:

array[0], array[1] = array[1], array[0]
hash[:a], hash[:b] = hash[:b], hash[:a]
point.x, point.y = point.y, point.x

[rrosenblum]

I think the issue stems from the cop seeing these as method calls instead of as assignments. I will try to dig into this cop to fix it.

[rrosenblum]

@alexdowad I may need some help with this one. There was a change in the code to start using NodePattern to check find_valid_order. I am not too familiar with NodePattern. I am sure I could figure it out, but you can probably fix the bug faster than I can. My first thought was to reintroduce the swapping_variables? which was replaced by find_valid_order.

[alexdowad]

@rrosenblum, no problem. I am just going for dinner now, but will look at it later.

[alexdowad]

My first thought was to reintroduce the swapping_variables? which was replaced by find_valid_order

swapping_variables? was replaced for good reason. It was only able to identify very basic cases where parallel assignment cannot validly be replaced. find_valid_order is far more robust. It just needs to be updated to handle aref assignments and attribute assignments.

[rrosenblum]

find_valid_order is far more robust. It just needs to be updated to handle aref assignments and attribute assignments.

After looking at the cop, that is what I was thinking. I had some guesses with how it would need to be modified, but I would need to play with it to really figure out how it needs to be changed.

[alexdowad]

Hey @rrosenblum... just opened a PR which fixes this one.

If you are interested, try reading the comments at the top of node_pattern.rb. The intention is that other developers should be able to pick up the "node pattern" language within 10 minutes -- but if there's anything which is hard to grasp, I will try to make the docs clearer.

[rrosenblum]

I just read over the docs at the top of node_pattern.rb, they seem detailed enough to me. The idea behind NodePattern makes sense to me. I will need to play around with it before I am completely comfortable with it.