Kotlin zipAll alternative - scala

Question

Does Kotlin have a function like .zipAll in Scala?
In Scala I can sum two array with different length using the zipAll function.
Scala:
val arrA = Array(1,2,3)
val arrB = Array(4, 5)
arrA.zipAll(arrB, 0, 0).map(x => x._1 + x._2)
Or what is the correct way to do this in Kotlin?

Answer 1

There is no in-build analog in Kotlin 1.0. It might be a good idea to add it into the stdlib. Feel free to file an issue on the YouTrack

Answer 2

Here is zipAll for Kotlin:
fun <T1: Any, T2: Any> List<T1>.zipAll(other: List<T2>, emptyValue: T1, otherEmptyValue: T2): List<Pair<T1, T2>> {
val i1 = this.iterator()
val i2 = other.iterator()
return generateSequence {
if (i1.hasNext() || i2.hasNext()) {
Pair(if (i1.hasNext()) i1.next() else emptyValue,
if (i2.hasNext()) i2.next() else otherEmptyValue)
} else {
null
}
}.toList()
}
And a unit test:
#Test fun sumTwoUnevenLists() {
val x = listOf(1,2,3,4,5)
val y = listOf(10,20,30)
assertEquals(listOf(11,22,33,4,5), x.zipAll(y, 0, 0).map { it.first + it.second })
}
And the same could be applied to arrays, other collection types, sequences, etc. An array-only version would be easier since you can index into the arrays. The array version could be:
fun <T1: Any, T2: Any> Array<T1>.zipAll(other: Array<T2>, emptyValue: T1, otherEmptyValue: T2): List<Pair<T1, T2>> {
val largest = this.size.coerceAtLeast(other.size)
val result = arrayListOf<Pair<T1, T2>>()
(0..this.size.coerceAtLeast(other.size)-1).forEach { i ->
result.add(Pair(if (i < this.size) this[i] else emptyValue, if (i < other.size) other[i] else otherEmptyValue))
}
return result.filterNotNull()
}
It returns a List because map function is going to turn you into a list anyway.

Answer 3

I made a quick tail-recursive version for fun. Not very efficient though, due to the list appends.
fun <T, U> List<T>.zipAll(that: List<U>, elem1: T, elem2: U): List<Pair<T, U>> {
tailrec fun helper(first: List<T>, second: List<U>, acc: List<Pair<T, U>>): List<Pair<T, U>> {
return when {
first.isEmpty() && second.isEmpty() -> acc
first.isEmpty() -> helper(first, second.drop(1), acc + listOf(elem1 to second.first()))
second.isEmpty() -> helper(first.drop(1), second, acc + listOf(first.first() to elem2))
else -> helper(first.drop(1), second.drop(1), acc + listOf(first.first() to second.first()))
}
}
return helper(this, that, emptyList())
}

Answer 4

This does not yet exist in the Koltin stdlib, but this is the suggested approach I posted in the youtrack ticket about this.
Here is a potential implementation modeled after the current zip function https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.collections/zip.html.
/**
* Returns a list of values built from the elements of `this` collection and the [other] collection with the same
* index using the provided [transform] function applied to each pair of elements. The returned list has length of
* the longest collection.
*/
fun <T, R, V> Iterable<T>.zipAll(
other: Iterable<R>,
thisDefault: T,
otherDefault: R,
transform: (a: T, b: R) -> V,
): List<V> {
val first = iterator()
val second = other.iterator()
val list = ArrayList<V>(maxOf(collectionSizeOrDefault(10),
other.collectionSizeOrDefault(10)))
while (first.hasNext() || second.hasNext()) {
val thisValue = if (first.hasNext()) first.next() else thisDefault
val otherValue =
if (second.hasNext()) second.next() else otherDefault
list.add(transform(thisValue, otherValue))
}
return list
}
// Copying this from kotlin.collections where it is an Internal function
fun <T> Iterable<T>.collectionSizeOrDefault(default: Int): Int =
if (this is Collection<*>) this.size else default
And here is how I use it
/**
* Takes two multiline stings and combines them into a two column view.
*/
fun renderSideBySide(
leftColumn: String,
rightColumn: String,
divider: String = " | ",
): String {
val leftColumnWidth: Int = leftColumn.lines().map { it.length }.maxOrNull() ?: 0
return leftColumn.lines()
.zipAll(rightColumn.lines(), "", "") { left, right ->
left.padEnd(leftColumnWidth) + divider + right
}
.reduce { acc, nextLine -> acc + "\n" + nextLine }
}
Example of how I am using this:
val left = """
Left Column
with some data
""".trimIndent()
val right = """
Right Column
also with some data
but not the same length
of data as the left colum.
""".trimIndent()
println(left)
Left Column
with some data
println(right)
Right Column
also with some data
but not the same length
of data as the left colum.
println(renderSideBySide(left,right))
Left Column | Right Column
with some data | also with some data
| but not the same length
| of data as the left colum.
println(renderSideBySide(right,left))
Right Column | Left Column
also with some data | with some data
but not the same length |
of data as the left colum. |