How do I idiomatically sum the size of multiple li...

# getting-started

[Erik]

How do I idiomatically sum the size of multiple lists that are nullable? E.g.

val totalSize = (list1?.size ?: 0) + (list2?.size ?: 0)

I'm wondering if there is a more idiomatic, easier to read syntax

[Erik]

Maybe

val totalSize = list1.orEmpty().size + list2.orEmpty().size

[Erik]

This doesn't allocate extra lists, since orEmpty references internal object EmptyList, which I use in many places, so it shouldn't incur an overhead, right?

[justin crow]

Would you first make a list of lists, then flatten it, then get the size? Something like

listOfNotNull(list1, list2, list3).flatten().size

[Erik]

That would allocate a new list unnecessarily. I'd like to sum the sizes directly, but without all the parentheses and question marks from my original question. I'll stick with the empty list solution

[justin crow]

Depends if you prefer clarity over efficiency I guess

[Erik]

True, but IMHO the clarity of adding sizes with a + operator is even better (than flattening a list of not nulls)

[Erik]

Thanks for your answer!

[Adam S]

a slight adjustment @justin crow, to avoid the flattening :)

listOfNotNull(list1, list2, list3).sumOf { it.size }

[justin crow]

Nice 👍

[Adam S]

if you want to try and de-deduplicate an operation, and make it prettier, you could define an extension property

private val List<*>?.sizeOrZero: Int get() = this?.size ?: 0

fun main() {
  val list1: List<Int>? = listOf(1, 2, 3)
  val list2: List<Int>? = listOf(1, 2, 3)
  val list3: List<Int>? = listOf(1, 2, 3)

  list1.sizeOrZero + list2.sizeOrZero + list3.sizeOrZero
}

[Adam S]

or a local function if the operation is only specific to a single function

fun main() {
  val list1: List<Int>? = listOf(1, 2, 3)
  val list2: List<Int>? = listOf(1, 2, 3)
  val list3: List<Int>? = listOf(1, 2, 3)

  fun List<*>?.sizeOrZero(): Int = this?.size ?: 0

  list1.sizeOrZero() + list2.sizeOrZero() + list3.sizeOrZero()
}

(it has to be a fun, not a val - a restriction that might be lifted in the future?)

[justin crow]

Nice, I’m new to Kotlin and didn’t realise you could have local extension functions

[Erik]

Thanks Adam. Those are some nice extensions. Now it's a matter of choosing between sticking with the stdlib (.orEmpty().size) or an extension. In my case I'd put the extension in some common source, but that would pollute the namespace a bit too (not that this is a very specific extension, but more generally applicable). I'll have to discuss with my reviewers which of the two approaches would fit better in our code bases.