Kotlin mutableMap foreach

Question

My question is more to understand the documentation. It say's:

fun <K, V> Map<out K, V>.forEach(
action: (Entry<K, V>) -> Unit)

However I don't understand how to implement it. How I get the Key and Value inside the loop?

I want to sum for each item in listItems the value of the price. The map associates a string with an item

data class Item(val name: String, val description: String, val price: String, val index: Int)

Imagine listItems contains this:

listItems["shirt"]-> name: shirt, description: lorem ipsum, price: 10, index: 0

listItems["shoes"]-> name: shoes, description: lorem ipsum, price: 30, index: 0

So the code would be something like:

var total: Int = 0
listItems.forEach {
       total += parseInt(value.price)
    }

However I don't understand how to acces this value refering to the V of the documentation

Answer 1

The lambda that you pass to forEach { ... } accepts an Entry<K, V>, and you can work with this entry, accessing its value:

listItems.forEach { total += parseInt(it.value.price) }

Which is equivalent to this using explicit lambda parameter:

listItems.forEach { entry -> total += parseInt(entry.value.price) }

Or, since Kotlin 1.1, you can use destructuring in lambdas:

listItems.forEach { (_, value) -> total += parseInt(value.price) }

---

If you only need to sum the values, you can use sumBy { ... }:

val total = listItems.entries.sumBy { parseInt(it.value.price) }

Answer 2

For the some of a collection, instead of using forEach, I'd suggest going for sumBy or fold.

You can get the underlying values by calling value on the Map<K,V>. So to get the sum you'd do something like this:

val total = listItems.values.sumBy{ it.price.toInt() }

Now there's no need to introduce a mutable var.