CODEWITHSUNDEEP
python-3.xfilterkotlinlist-comprehension
horcle_buzz
horcle_buzz

Reputation: 2161

Nested comprehension in Kotlin

Suppose I have the following nested for loop:

val test = mutableSetOf<Set<Int>>()
for (a in setA) {
    for (b in setB) {
        if (a.toString().slice(2..3) == b.toString().slice(0..1)) {
            test.add(setOf(a,b))

        }
    }
}

In python, I could do a simple comprehension as

test = {[a,b] for a in setA for b in setB if a.str()[2:3] == b.str[0:1]}

I'm having a helluva time converting this to Kotlin syntax. I know for a single for loop with a conditional, I could use a filter and map to get the desired results (using the idiom: newSet = oldSet.filter{ conditional }.map { it }, but I cannot for the life of me figure out how to do the nesting this way.

Upvotes: 3

Views: 703

Answers (2)

Jayson Minard
Jayson Minard

Reputation: 85946

If Kotlin doesn't have it, add it. Here is a cartesian product of the two sets as a sequence:

fun <F,S> Collection<F>.cartesian(other: Collection<S>): Sequence<Pair<F,S>> =
    this.asSequence().map { f -> other.asSequence().map { s-> f to s } }.flatten()

Then use that in one of many ways:

// close to your original nested loop version:

setA.cartesian(setB).filter { (a,b) ->
    a.toString().slice(2..3) == b.toString().slice(0..1)
}.forEach{ (a,b) -> test.add(setOf(a,b)) }


// or, add the pair instead of a set if that makes sense as alternative

setA.cartesian(setB).filter { (a,b) ->
    a.toString().slice(2..3) == b.toString().slice(0..1)
}.forEach{ test2.add(it) }

// or, add the results of the full expression to the set at once

test.addAll(setA.cartesian(setB).filter { (a,b) ->
                  a.toString().slice(2..3) == b.toString().slice(0..1)
            }.map { (a,b) -> setOf(a,b) } )

// or, the same as the last using a pair instead of 2 member set

test2.addAll(setA.cartesian(setB).filter { (a,b) ->
        a.toString().slice(2..3) == b.toString().slice(0..1)
    })

The above examples use these variables:

val test = mutableSetOf<Set<Int>>()
val test2 = mutableSetOf<Pair<Int,Int>>()
val setA = setOf<Int>()
val setB = setOf<Int>()

Upvotes: 0

creativecreatorormaybenot
creativecreatorormaybenot

Reputation: 126684

This is what IDEA proposes:

for (a in setA)
    setB
            .filter { a.toString().slice(2..3) == it.toString().slice(0..1) }
            .mapTo(test) { setOf(a, it) }

I do not think there is much to do about it. I think their is no native approach that is similar to the Python one, but it already actually is in terms of length very similar because only the functions and their names make it that long.

If we take a look a this hypothetical example:

for (a in setA) setB.f { a.t().s(2..3) == it.t().s(0..1) }.m(test) { setOf(a, it) }

It is not far from the Python example. The Python syntax is just very different.

(functions for that hypothesis)

fun <T> Iterable<T>.f(predicate: (T) -> Boolean) = filter(predicate)

fun String.s(range: IntRange) = slice(range)

fun <T, R, C : MutableCollection<in R>> Iterable<T>.m(destination: C, transform: (T) -> R) = mapTo(destination, transform)

fun Int.t() = toString()

Upvotes: 2

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