How do I use del correctly in ireduce block?

[zozowell]

I reported this as a bug already, but it's a good chance to be a error usage either. So, I cross-post in the discussion; I would appreciate it if anyone could shed some light on this weird behavior. Thanks!

Describe the bug
When I use delete operator in the ireduce block, the del doesn't work as I expected.

Version of yq: 4.44.3
Operating system: mac
Installed via: homebrew

Input Yaml
test.yml:

- a: cat
- b: dog
- c: bat

Command
The command you ran:

A="[cat, bat]" yq 'env(A)[] as $a ireduce(.; del(.[] | select(.[] == $a)))' test.yaml

Actual behavior

bash-3.2$ A="[cat, bat]" yq 'env(A)[] as $a ireduce(.; del(.[] | select(.[] == $a)))' test.yaml
- b: dog
- c: bat

Expected behavior

bash-3.2$ A="[cat, bat]" yq 'env(A)[] as $a ireduce(.; del(.[] | select(.[] == $a)))' test.yaml
- b: dog

Additional context
The behavior is very weird as the input variable A changed; another strange behavior is that: I can't delete all the elements, even deleting the array index 0 every time.

bash-3.2$ A="[cat, bat, dog]" yq 'env(A)[] as $a ireduce(.; del(.[0]))' test.yaml
- b: dog

[mikefarah]

I wouldn't use del with ireduce - you're deleting from the array as you are iterating through it. Best practice is to use ireduce to create a new array or map.

There are a lot of other ways to do this, it's made a little harder because you have an array of maps, but for example:

A="[cat, bat]" ./yq '.[] | to_entries[] | select([.value] as $v | env(A) | contains($v) == false) | [.] | from_entries | [.]' examples/data1.yaml

• High level, this converts the data structure to an array of entries (key, values);
• filters out any that are contained in the A environment variable.
• Converts the entries array back into an array of maps.

[zozowell]

@mikefarah, thanks a lot for your reply.

you're deleting from the array as you are iterating through it

I understand your point. However, this is not what I'm doing in my expression, I think. I'm iterating over an array A, then doing one del on another array B in every iteration. it doesn't do multiple dels in one iteration at all. I suppose it is simply equal to pipe up multiple object/array del ops?

And, the current reduce behavior is a bit contradictory with the "map-reduce" common semanteme. The accumulator shall be reset to a whole new value after each iteration. The values between two reduce iteration shall have no ties. WDYT?

Thanks for creating yq. 💯