Is there a way to use `putAll` on a mutable map, p...

# codereview

[Klitos Kyriacou]

Is there a way to use putAll on a mutable map, passing it another map and a value transformation function? putAll allows you to put all entries of another map into this map, but doesn't let you transform them on the fly. mapTo allows you to transform the entries but you can only write them to a Collection, not to a MutableMap. Is there a method that does what the commented-out line does?

fun main() {
    val map1 = mapOf(1 to "one", 2 to "two", 3 to "three")
    val map2 = buildMap {
        put(4, "fourfour")
        put(5, "fivefive")

        // map1.mapTo(this) { (k, v) -> k to v + v } DOESN'T COMPILE

        // instead we have to use:
        putAll(map1.toMutableMap().apply { replaceAll { _, v -> v + v } })
        // or:
        putAll(map1.map { (k, v) -> k to v + v })
    }
    println(map2)
}

[Paul Griffith]

putAll(map1.mapValues { (_, v) -> v + v })

?

[Klitos Kyriacou]

Thanks, Paul. Your suggestion has led me to exactly what I'm looking for: mapValuesTo.

map1.mapValuesTo(this) { (k, v) -> v + v }

[Paul Griffith]

Yeah, just realized you were more asking for direct mutation of the temp buildMap backing map