Kotlin Map 和计算函数

Question

I am trying to re-compute values for all entries in a mutable map without changing the keys.

Here is the code (with comments showing what I am expecting)

fun main(args: Array<String>) {

    var exampleMap: MutableMap<Int, String> = mutableMapOf(1 to "One", 2 to "Two")

    // Before it's {1-"One", 2-"Two"}
    with(exampleMap) {
        compute(k){
            _,v -> k.toString + "__" + v
        }
    }

    // Expecting {1->"1__One", 2->"2__Two"}


 }

I don't want to use mapValues because it creates a copy of the original map. I think compute is what I need, but I am struggling to write this function. Or, rather I don't know how to write this. I know the equivalent in Java (as I am a Java person) and it would be:

map.entrySet().stream().forEach(e-> map.compute(e.getKey(),(k,v)->(k+"__"+v)));

How can I achieve that in Kotlin with compute ?

Regards,

Answer 1

The best way to do this is by iterating over the map's entries . This ensures you avoid any potential concurrent modification issues that might arise due to modifying the map while iterating over it.

map.entries.forEach { entry ->
    entry.setValue("${entry.key}__${entry.value}")
}

Answer 2

Use onEach function:

var exampleMap: MutableMap<Int, String> = mutableMapOf(1 to "One", 2 to "Two")
println(exampleMap)
exampleMap.onEach {
   exampleMap[it.key] = "${it.key}__${it.value}"     
}
println(exampleMap)

Output:

{1=One, 2=Two}
{1=1__One, 2=2__Two}

onEach performs the given action on each element and returns the list/array/map itself afterwards.