Hi there Are there any functional fun to iterate over a list kotlinlang #announcements

Hi there! Are there any functional fun to iterate over a list and apply an operation to the current element with the previous one? It would be like a fold, but instead of accumulating the result, just applying it to each element. E.g:

(1..9).fold("a", { last, current -> last + current })

This just returns a123456789 but I actually need to create the following list: { "a1", "a12", "a123", "a1234", .. } I would be more like a map but knowing previous element:

(1..9).map { "$previousElement$it" }

streetsofboston

12/12/2019, 2:52 PM

Not sure if this fun exists on a List, but have you tried

scan

? (this fun exists on Rx)

AdalPari

12/12/2019, 2:54 PM

Not really, will check!

Kroppeb

12/12/2019, 3:47 PM

This is a scan, which doesn't exist in the stdlib sadly

Hampus Londögård

12/12/2019, 4:15 PM

You can use

windowed

(1..9).windowed(size=2).map { items -> "${items.first()}${items.last()}" }

It defaults to

partialWindows=false

. Not really what you asked for though, but it does the job 🙂

Kroppeb

12/12/2019, 4:17 PM

I don't see how you could use it.

Hampus Londögård

12/12/2019, 4:17 PM

Nvm, I got it wrong. Thought it was just the previous one

Kroppeb

12/12/2019, 4:20 PM

var previous = "a"
(1..9).map {current -> 
    previous += current
    previous
})

Hampus Londögård

12/12/2019, 4:22 PM

This works

(1..9).fold(listOf<String>()) { acc, current -> acc + ((acc.lastOrNull() ?: "") + current) }

👍 1

streetsofboston

12/12/2019, 4:22 PM

^^^ @Kroppeb That’ll work for ordered and indexed collections, but you are using a side-effect by updating an external `var`….

☝️ 1

Kroppeb

12/12/2019, 4:26 PM

This is the

scan

implementation I have in my utils for AoC. It doesn't return the start value. If you want the start value to also be returned you can change line 3 to

val ret = mutableListOf(start)

Untitled

👍 3

streetsofboston

12/12/2019, 4:52 PM

Here’s another version of

scan

, which can handle large collections and scans the in- and out-put values one at a time, avoiding the allocation of possibly large collections until the end.

fun <T, R> Sequence<T>.scan(start: R, oper: (R, T) -> R): Sequence<R> = sequence {
    var next = start
    for (value in iterator()) {
        next = oper(next, value).also { yield(it) }
    }
}

fun <T, R> Iterable<T>.scan(start: R, oper: (R, T) -> R): Iterable<R> =
    this.asSequence().scan(start, oper).asIterable()

It uses a

Sequence

to do so.

AdalPari

12/12/2019, 5:05 PM

Lot of answers! thanks!

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