I m wondering if there is something that works like `zip` bu

Question

I m wondering if there is something that works like `zip` but instead of returning pairs based on index it returns pairs based on a predicate For example ``` data class A val id String data class B val id String val listA List lt A gt = listOf A 3 A 8 val listB List lt B gt = listOf B 8 B 2 B 4 val pairs List lt Pair lt A B gt gt = listA someTransformation listB it id == id pairs would contain listOf A 8 B 8 ``` Would love to know if there s a best practice around this type of operation

Justin · Answer

And to be clear there could be several matching id s between the lists even though there s only one in my example

karelpeeters · Answer

What do you want yo happen if there are multiple

Justin · Answer

Oh sorry there will never be multiple matches in listB for a single item in listA Ids would only appear once per list When I said multiple I meant like if listB had an additional element `B 3 ` matching the first element in listA

Justin · Answer

Does that make sense

Justin · Answer

And in my scenario there s more than one property in each data class that I m matching them against ruling out the simple sort then zip approach that would work in my hypothetical

Jason · Answer

Like `intersect` that takes a block for the equality test

Jason · Answer

Kinda

Justin · Answer

Yea kinda

karelpeeters · Answer

Cartesian product + filter should do it unfortunately there s no cartesian product in Kotlin see <https youtrack jetbrains com issue KT 13012> and related issues You ll have to write it yourself ```listA flatMap a gt listB map b gt a to b filter a b gt f a b ``` maybe with a `asSequence ` inbetween

karelpeeters · Answer

Another possibility ```listA flatMap a gt listB mapNotNull b gt if f a b a to b else null ```

Justin · Answer

wow thanks < karelpeeters> let me try these and see if they do what i need

Justin · Answer

what is the `f` in the first one i e ` f a b `

Jason · Answer

`a id == b id`

Justin · Answer

oh i see

Justin · Answer

what do you mean by with a `asSequence ` in between

Alowaniak · Answer

`listB asSequence map ` so that it doesn t actually create a B long list for each `a` which gets mostly `filter`ed out after it

Justin · Answer

for the first one i get a type error in the filter on b

Justin · Answer

it thinks b is a Pair lt A B gt not B

Jason · Answer

``` gt gt gt listA flatMap a gt listB map b gt a to b filter a b gt a id == b id res45 kotlin collections List lt kotlin Pair lt Line 10 A Line 9 B gt gt = A id=8 B id=8 ```

Justin · Answer

ah that works i was wrapping the predicate part in still newish to kotlin thank you both very much

Jason · Answer

+1