is there a better way to turn a List of Maps into ...

# getting-started

[Bernhard]

is there a better way to turn a List of Maps into a single map with all of their values combined on the same index than this:

val increaseSkills: List<Map<KingdomSkill, Set<KingdomSkill>>> = emptyList()
    val groupedSkills = mutableMapOf<KingdomSkill, Set<KingdomSkill>>()
    increaseSkills.forEach { map ->
        map.forEach { (skill, skills) ->
            groupedSkills.put(skill, (groupedSkills[skill] ?: emptySet()) + skills)
        }
    }

[Wout Werkman]

Something in the lines of .flatMap { it.entries }.toMap()?

[Bernhard]

oh, you're totally right

[Bernhard]

but the entries need to be grouped by key and values need to be combined

[Bernhard]

ok, I think I can manage that

[Bernhard]

thank you!

[Sam]

I think:

increaseSkills
    .flatMap { it.entries }
    .groupBy({ it.key }, { it.value })
    .mapValues { it.value.flatten().toSet() }

[Sam]

But sometimes the explicit way is more readable!

[Bernhard]

more readable but those places tend to have more bugs as well

[Bernhard]

didn't know the value selector and mapValues method yet, thank you

[Michael Krussel]

If you go with the original, I would look at using buildMap instead of declaring a mutableMap.

And I find this a little cleaner groupedSkills[skill].orEmpty() + skills than (groupedSkills[skill] ?: emptySet()) + skills

But both of those are really just preferences.

[Bernhard]

thank you, .orEmpty() is great

[Bernhard]

I need to autocomplete on nullable types more often

[michaelsims]

increaseSkills.reduce() { acc, map -> map + acc.mapValues { (k, v) -> v + map[k].orEmpty() } }

[Bernhard]

last lambda is the conflict/merge function?

[michaelsims]

Yes

[Bernhard]

thanks, that's the best one so far 🙂

[Bernhard]

can't find combine, is that only available on the jvm?

[michaelsims]

Ouch, this is embarrassing. I seem to have used a function from Arrow w/o realizing 😬

[Bernhard]

Monoid one?

[michaelsims]

it's from arrow-core

[michaelsims]

I'll do another one that doesn't use arrow shortly 😄

[Bernhard]

heh

[Bernhard]

yeah, I was looking for some combine/merge method/function

[Bernhard]

in Java there's Map.merge

[michaelsims]

Not as pretty, but:

increaseSkills.reduce() { acc, map -> map + acc.mapValues { (k, v) -> v + map[k].orEmpty() } }

[Bernhard]

thanks