How add a list inside a map in kotlin

Question

I need to add some MutableList<String> to a map Map<String, List<String>>, here is how I tried to initialize it :

    private var theSteps: MutableList<String> = mutableListOf()
    private var optionsList: Map<String, List<String>> = mapOf()

Then I add data to the `MutableList this way :

        theSteps.add("one")
        theSteps.add("two")
        theSteps.add("three")

everything works fine untill i try to add to the Map:

optionsList.add("list_1" to theSteps)

It just gives me the error Unresolved reference add and I can't find clear documentation about how to add items to it.

score 4 · Accepted Answer · answered Dec 26 '19 at 10:39

optionsList must be a MutableMap to add anything, just like you have a MutableList; or you can use

theSteps += "list_1" to theSteps

to create a new map with the added pair and update theSteps variable. This calls the plus extension function:

Creates a new read-only map by replacing or adding an entry to this map from a given key-value pair.

(search for the above to get to the correct overload)

Ori Marko · Answer 2 · 2019-12-26T10:46:47.960

You can't add to your map because mapOf is creating a read-only map

fun <K, V> mapOf(): Map<K, V>
Returns an empty read-only map.

You probably want to create a MutableMap (or similar)

private var optionsList: Map<String, List<String>> = mutableMapOf()

You can then use plus method:

optionsList = optionsList.plus("list_1" to theSteps)

Or see other options as @voddan:

val nameTable = mutableMapOf<String, Person>()    
fun main (args: Array<String>) {
    nameTable["person1"] = example

2 Answers2