There is no `lastIndex` (computed property) for `C...

# stdlib

[Thomas]

There is no lastIndex (computed property) for Collection in `Collections.kt`; lastIndex is available for List.

I see no reason why Collections cannot have lastIndex, when:
1. size is a property declared by Collection, so there is no issue with implementation, which is simply size - 1.
2. indices which calls 0..size - 1 under the hood is available for Collections, so we have no issue with "last index might be something other than size-1".

Could there be something that I didn't consider?

[jw]

What are you going to do with it? Collections are not intrinsically indexable nor can you look up elements by index.

[Thomas]

but then why do we have the computed property indices on Collections?

[jw]

It's certainly less useful than on collections which support random access, but you can still use it as a shortcut to the ordinal values which correspond to the elements in a collection. Basically as a replacement for mapIndexed { _, index -> index }. I have definitely used indices in the past for non-random access purposes, although I can't find specific examples right now.

[Klitos Kyriacou]

All Iterable types, including Collection, have the method elementAt(index).

[Thomas]

The more I think of it, the more I think that indices should not be declared at Collection

[Sam]

Yeah, that does seem weird and confusing. I can't think of a use case for indices on an unordered collection like a set.

[Sam]

@jw would be very interested if you end up remembering what any of those examples were!

[jw]

Here's some I found via grep. This is only applying the map operation, but I expect that to be the most common in the case where you want to map the ordinals to something.

./cashapp/redwood/redwood-tooling-codegen/src/main/kotlin/app/cash/redwood/tooling/codegen/widgetProtocolGeneration.kt:318:                        beginControlFlow("{ %L ->", trait.parameterTypes.indices.map { CodeBlock.of("arg$it") }.joinToCode())

./aosp/androidx-main/emoji2/emoji2-emojipicker/src/main/java/androidx/emoji2/emojipicker/PopupViewHelper.kt:134:            Layout.FLAT -> arrayOf(variants.indices.map { it + 1 }.toIntArray())

./aosp/androidx-main/sqlite/sqlite-inspection/src/androidTest/java/androidx/sqlite/inspection/test/GetSchemaTest.kt:194:                alreadyOpenDatabases.indices.map { testEnvironment.receiveEvent().databaseOpened }
./aosp/androidx-main/sqlite/sqlite-inspection/src/androidTest/java/androidx/sqlite/inspection/test/TrackDatabasesTest.kt:91:            val actual = expected.indices.map { testEnvironment.receiveEvent().databaseOpened }

./kotlin/kotlin/compiler/tests/org/jetbrains/kotlin/generators/tests/GenerateInRangeExpressionTestData.kt:80:        val valNames = expressions.indices.map { "$prefix$it" }

[jw]

it's still running but you get the idea of how it can be useful to take a collection of things and map it to a list of other things based solely on the ordinals of the elements. you can achieve the very same with (0 until size).map, of course, but might as well use what you got

[Sam]

Or List(size) { … }, but I can see the appeal of using indices too. Thanks for sharing!