Kotlin iterate a collection and map a certain value to new map - kotlin

Question

I have a map of collections . I need to get a list of ids from that..
val m1 = mapOf("id" to 1, "name" to "Alice")
val m2 = mapOf("id" to 2, "name" to "Bob")
val m3 = mapOf("id" to 3, "name" to "Tom")
val nameList = listOf(m1, m2, m3)
The result shall be [1, 2, 3]

Answer 1

Assuming you want a list as per the example, not a map as per the title, I would do it like this:
val result = nameList.map {
it.getValue("id").also { id ->
require(id is Int) { "id must be an Int" }
} as Int
}
This has the advantage of handling the following errors cleanly:
The id key is missing: NoSuchElementException: Key id is missing in the map
The id value is not an Int: IllegalArgumentException: id must be an Int

Answer 2

First things first, I believe that if you can, you should use classes instead of maps for storing heterogeneous data like this. So instead of your maps, you can use:
data class Person(val id: Int, val name: String)
val m1 = Person(id = 1, name = "Alice")
val m2 = Person(id = 2, name = "Bob")
val m3 = Person(id = 3, name = "Tom")
val list = listOf(m1, m2, m3)
val idsList = list.map { it.id } // no error handling required, rely on the type system
Now, if you really want to use maps like that, you have several options.
If you're certain the id key will be present and its value will be an Int, you can use the following:
nameList.map { it["id"] as Int }
This will fail with NullPointerException if id is not present in one of the maps or with ClassCastException if it's not an Int.
Normally you should make sure your map matches your contract at creation time, and not when accessing this kind of information.
But if you need to handle errors here for some reason, you can use the following instead:
nameList.map {
(it.getValue("id") as? Int) ?: error("'id' is not an Int")
}
getValue fails on absent keys with NoSuchElementException, and the error() call fails with IllegalStateException. You can also use other kinds of exceptions using throw or require().
If you want to just ignore the entries that don't have a valid integer id, you can use the following:
nameList.mapNotNull { it["id"] as? Int }
If you want to ignore the entries that don't have an id, but fail on those who have a non-integer id, you can use this:
nameList.mapNotNull { map ->
map["id"]?.let { id ->
(id as? Int) ?: error("'id' is not an Int")
}
}
These 2 last examples rely on mapNotNull, which filters the elements out if their mapped value is null.