I’m looking into the std lib functions for Collect...

# announcements

[Julio Zynger]

I’m looking into the std lib functions for Collections, is there a function to get the first index of match to a predicate in the Iterable return null if not found?
In other words, I want to avoid a double pass on the Collection by using myCollection.indexOf(myCollection.find { predicate(it) }) (plus avoid the handling of -1 as a result type.

[Julio Zynger]

For now we’re bundling this extension, but I wonder if the stdlib already provides a similar thing (I’d be happy to open a PR if not)
https://dev.to/amay077/gets-first-index-of-match-to-condition-in-list-6c7

[SackCastellon]

You can use indexOfFirst.
See: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/index-of-first.html

[SackCastellon]

However, this function still returns -1 if no element matches the predicate

[Julio Zynger]

Yes, correct. I’d like to avoid that extra hop, if possible. So, in the callsite, i’d like to be able to write

val index: Int = myCollection.theMagicFunctionHere { myPredicate(it) } ?: somethingElse()

[spand]

I dont think so. I made my own last time I needed it

[Dias]

that's very unusual thing: returning index as a null. Don't think kotlin will ever have it

[Dias]

why do you need non existing index be anything else other than -1?

[SackCastellon]

You could use takeIf { it != -1 } to transform a -1 to null

Or create an extension function like:

inline fun <T> Iterable<T>.indexOfFirstOrNull(predicate: (T) -> Boolean): Int? = indexOfFirst(predicate).takeIf { it != -1 }

[Julio Zynger]

Thanks, yes that extension function would suffice, or the one I posted the link of as the second message of the thread. It seems like kotlin doesn’t provide it out of the box, and @Dias raises a good point re:predictability of the API as well.