Hey guys. I want a combination of find and map in a single function. Find a first element that satisfies a predicate and map it over function at the same time. Scala has

collectFirst

for that. How to do it in Kotlin ?

You could use

mapNotNull

with your partial transformation function and then

firstOrNull

on the result.

You can write your own extension function

collectFirst

If you only have a partial function, no associated predicate and you want to evaluate this lazily, then I think you'll have to write your own method.

You can try to play with a sequence to avoid intermediate result, something like that:

list.asSequence().filter { }.map {  }.first()

The generic implementation of

collectFirst

is also not hard to write, for example like this:

fun <T, R> Iterable<T>.collectFirst(transform: (T) -> R?) : R? {
    for (i in this) {
        val o = transform(i)
        if (o != null) return o
    }
    return null
}

Or for example:

list
    .find { it % 2 == 0 }
    ?.let { abs(it) }

If you need separate lambdas for predicate and transformation, then one function call will not look nice:

list.collectFirst({ it % 2 == 0 }, { abs(it) })

@Pavlo Liapota I would also recommend to split the function into predicate and transformation, because that will be more clear and will allow the use of existing library functions. But the OP specifically asked for an analogon of a Scala library method and that method only takes one partial function as its argument, so I replicated that behaviour.

Thank you guys. I think @murphy version is exactly what I’m looking for. The thing is that I don’t wanna recompute value, since filtering function already does the mapping, I just want this result.

@murphy Ah, I see, sorry, I am not familiar with Scala 🙂 @Бежан Александр You can also make that extension function

inline

.

The solution using

asSequence

before

mapNotNull

and

firstOrNull

instead of a dedicated extension method would also avoid computing too many intermediate results.

Yeah, this is also good

Sure, if this functionality is used frequently an extension function definitely makes sense.

@Pavlo Liapota thanks for inline suggestion