Changing the value in Map-CodePudding

I'm studying kotlin. Now I'm solving a problem: if there is a key in the map, then substract one, and if there is not, then put 10. I wrote the code, but maybe it can be simplified without using an intermediate variable?

    val map = mutableMapOf<String, String>()

    val value = map["key"]
    map["key"] = if (value != null) (value.toInt()-1).toString() else "10"

CodePudding user response：

For Kotlin/JVM, you can use compute, and use this really nice nullable chaining in the lambda:

map.compute("key") { _, value ->
    // value would be null if the key doesn't exist in the map
    // so we chain the null all the way until we get the final string
    value?.toInt()?.minus(1)?.toString() ?: "10"
}

Though I would also suggest that you use a MutableMap<String, Int> instead, to avoid all the intermediate toInt and toString mess. Then you can just do:

map.compute("key") { _, value ->
    value?.minus(1) ?: 10
}

If you want a strings map later on, just mapValues.

CodePudding user response：

for a better performance i recommend using fastutil

add this to your pom file

    <dependency>
        <groupId>it.unimi.dsi</groupId>
        <artifactId>fastutil</artifactId>
        <version>8.1.1</version>
    </dependency>

then in case, there isn't any key your put 10 else you subtract 1

import it.unimi.dsi.fastutil.objects.Object2IntOpenHashMap

    fun main() {
        val key = "key"
        val map = Object2IntOpenHashMap<String>()
        if (map.putIfAbsent(key, 10) > 0) map.addTo(key, -1)
    }

CodePudding user response：

Your current code seems good to me. If you really want to avoid an intermediate variable, just replace value with map["key"] everywhere.

Other ways to do the same thing can be:

map["key"] = map["key"]?.toInt()?.minus(1)?.toString() ?: "10"
OR
map["key"] = map["key"].let { if (it != null) (it.toInt() - 1).toString() else "10" }

But I feel that the original code looks more readable than these.
Btw if the map contains only numbers, you can simply use a mutableMapOf<String, Int> and avoid the toInt() and toString() calls. And if it doesn't always contain numbers, you might want to use toIntOrNull() instead of toInt()

CodePudding user response：

You can use Map.putIfAbsent method.

Method description: if the specified key is not already associated with a value or is mapped to null associates it with the given value and returns null, else returns the current value:

map.putIfAbsent("key","10")?.let{map["key"]=(it.toInt()-1).toString()}

CodePudding user response：

I would do it like this

map["key"] = map["key"]?.let{(it.toInt() - 1).toString()} ?: "10"