Is there a Kotlin equivalent for Groovy's findIndexValues?

Question

Does Kotlin have something to filter a collection and return the matching indexes?

E.g. like Groovy's findIndexValues: http://docs.groovy-lang.org/latest/html/groovy-jdk/java/lang/Iterable.html#findIndexValues(groovy.lang.Closure)

Something like:

    fun  List.findIndexValues(predicate: (T) -> Boolean): List {
        var indexValues = mutableListOf()
        this.forEachIndexed { index, it ->
            if (predicate(it)) {
                indexValues.add(index)
            }
        }
        return indexValues
    }

user_4618331 · Answer

The simplest way I can think of to do this is to use mapIndexedNotNull:

fun  List.findIndexValues(predicate: (T) -> Boolean): List =
    mapIndexedNotNull { i, t -> i.takeIf { predicate(t) } }

I don't believe there's a function for this in the standard library.

user_15097422 · Answer

There are basically 2 simple ways according to me.

//say there is a List x of Strings
val x = listOf()

//I don't believe you are looking for this.
//string is the required String of x at index.
for ((index, string) in x.withIndex()) {
      TODO()
}

//2nd method is using filterIndexed

/**
 * Returns a list containing only elements matching the given predicate.
 * @Params: predicate - function that takes the index of an element and the element itself and returns the result of predicate evaluation on the element.
 */
x.filterIndexed { index, string ->
      TODO()
}

user_430128 · Answer

I like @Sam's answer, but I find this implementation to be slightly more readable as it filters explicitly on predicate as opposed to implicitly via null:

fun  List.findIndexValues(predicate: (T) -> Boolean): List =
    withIndex().filter { (_, t) -> predicate(t) }.map { it.index }

Is there a Kotlin equivalent for Groovy's findIndexValues?

3 Answers