How to use Kotlin fold function to convert an array into a map?

Question

I am trying to convert an Array via fold into an indexed Map. Somehow IntelliJ flags that when I return the accumulator that it expects Unit. When I remove the return it complains that I require the datatype I originally wanted to return.

The code is as follows (Item is just a data class)

    constructor(vararg items: Item){
    val itemMap = items.fold(mutableMapOf<Int, MutableList<Item>>(), { acc, item ->
        if (acc.containsKey(item.state)) {
            acc[item.state]?.add(item)
        } else {
            acc.put(item.state, mutableListOf(item))
        }
        return acc
    })

}

Its a bit late here so I probably miss something very obvious. Any help would be very appreciated.

Thanks

FWIW, you can also achieve this entire thing with `val itemMap = items.groupBy { it.state }`. — Oliver Charlesworth, Dec 26 '17 at 14:32
Thanks @OliverCharlesworth. This is a lot easier. I didn't know about the groupBy function. — Oliver, Dec 26 '17 at 15:56

hotkey · Answer 1 · 2017-12-26T15:23:31.983

Use the qualified return@fold operator instead of return. In Kotlin, return without a qualifier means 'return from the innermost fun (ignoring lambdas)'.

val itemMap = items.fold(mutableMapOf<Int, MutableList<Item>>(), { acc, item ->
    if (acc.containsKey(item.state)) {
        acc[item.state]?.add(item)
    } else {
        acc.put(item.state, mutableListOf(item))
    }
    return@fold acc
})

See Whats does “return@” mean?, Return at Labels in the language reference.

Or just use the result expression, omitting return:

val itemMap = items.fold(mutableMapOf<Int, MutableList<Item>>(), { acc, item ->
    if (acc.containsKey(item.state)) {
        acc[item.state]?.add(item)
    } else {
        acc.put(item.state, mutableListOf(item))
    }
    acc
})

Basically, this kind of fold is implemented in the standard library: see .groupBy { ... }.

How to use Kotlin fold function to convert an array into a map?

1 Answers1