kotlin-stdlib/kotlin.collections/foldTo

foldTo

inline fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.foldTo(destination: M, initialValueSelector: (key: K, element: T) -> R, operation: (key: K, accumulator: R, element: T) -> R): M(source)

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in the given destination map. An initial value of accumulator is provided by initialValueSelector function.

Since Kotlin

1.1

Return

the destination map associating the key of each group with the result of accumulating the group elements.

Parameters

initialValueSelector

a function that provides an initial value of accumulator for each group. It's invoked with parameters:

  • key: the key of the group;

  • element: the first element being encountered in that group.

If the destination map already has a value corresponding to some key, that value is used as an initial value of the accumulator for that group and the initialValueSelector function is not called for that group.

operation

a function that is invoked on each element with the following parameters:

  • key: the key of the group this element belongs to;

  • accumulator: the current value of the accumulator of the group;

  • element: the element from the source being accumulated.

Samples


fun main() { 
   //sampleStart 
   val fruits = listOf("cherry", "blueberry", "citrus", "apple", "apricot", "banana", "coconut")

val evenFruits = fruits.groupingBy { it.first() }
    .foldTo(mutableMapOf(), { key, _: String -> key to mutableListOf<String>() },
            { _, accumulator, element ->
                if (element.length % 2 == 0) accumulator.second.add(element)
                accumulator
            })

val sorted = evenFruits.values.sortedBy { it.first }
println(sorted) // [(a, []), (b, [banana]), (c, [cherry, citrus])]

evenFruits.clear() // evenFruits is a mutable map 
   //sampleEnd
}
inline fun <T, K, R, M : MutableMap<in K, R>> Grouping<T, K>.foldTo(destination: M, initialValue: R, operation: (accumulator: R, element: T) -> R): M(source)

Groups elements from the Grouping source by key and applies operation to the elements of each group sequentially, passing the previously accumulated value and the current element as arguments, and stores the results in the given destination map. An initial value of accumulator is the same initialValue for each group.

If the destination map already has a value corresponding to the key of some group, that value is used as an initial value of the accumulator for that group.

Since Kotlin

1.1

Return

the destination map associating the key of each group with the result of accumulating the group elements.

Parameters

operation

a function that is invoked on each element with the following parameters:

  • accumulator: the current value of the accumulator of the group;

  • element: the element from the source being accumulated.

Samples


fun main() { 
   //sampleStart 
   val fruits = listOf("apple", "apricot", "banana", "blueberry", "cherry", "coconut")

// collect only even length Strings
val evenFruits = fruits.groupingBy { it.first() }
    .foldTo(mutableMapOf(), emptyList<String>()) { acc, e -> if (e.length % 2 == 0) acc + e else acc }

println(evenFruits) // {a=[], b=[banana], c=[cherry]}

evenFruits.clear() // evenFruits is a mutable map 
   //sampleEnd
}