In Swift, is there a builtin method for splitting an array into a prefix and a suffix?

Question

Is there a builtin method on Swift Arrays which splits it into two pieces, preserving the order of all elements?

Something akin to Array.prefix and Array.suffix, combined into one?

I'm aware of partition and split, but they don't preserve order and size, respectively.

Example:

[1,2,3,5,6,2,3,5].cut(where: { $0 < 5 })
>>> ([1,2,3], [5,6,2,3,5])

@matt In this particular case I'm looking for a function with a predicate, as in the example I included. — SwiftsNamesake, Jan 19 '20 at 02:34

Alexander · Accepted Answer · 2020-01-19T14:23:16.483

5

I'm afraid there isn't such a function, which is a shame, because I've needed it several times now. It's pretty easy to roll your own, though:

extension RangeReplaceableCollection {
    func cut(where belongsInFirstHalf: (Element) -> Bool) -> (SubSequence, SubSequence) {
        guard let splittingIndex = self.firstIndex(where: { !belongsInFirstHalf($0) }) else {
            return (self[...], SubSequence())
        }

        return (
            self[..<splittingIndex],
            self[splittingIndex...]
        )
    }
}

print([1,2,3,5,6,2,3,5].cut(where: { $0 < 5 })) // => (ArraySlice([1, 2, 3]), ArraySlice([5, 6, 2, 3, 5]))

edited Jan 19 '20 at 14:23

answered Jan 19 '20 at 02:15

Alexander

59,041
12
98
151

Yup, that's what I ended up doing. Thanks for your answer :) – SwiftsNamesake Jan 19 '20 at 02:35
Hey I’ve never seen `self[...]` before. – matt Jan 19 '20 at 02:37
I don't think the word `where` is meaningful here. What would the `$0` be? Maybe `beforeFirstWhere`? – Jan 19 '20 at 04:12
@matt https://stackoverflow.com/a/31782490/2303865 and https://stackoverflow.com/a/47116623/2303865 learned that trick from Hamish more than 2 years ago – Leo Dabus Jan 19 '20 at 05:33
1

Why not simply `return (self[.. – Leo Dabus Jan 19 '20 at 05:40
@LeoDabus Oooooo I forgot about those! – Alexander Jan 19 '20 at 14:22
@matt The weird thing is that `...` isn't an operator, since Swift requires all operators to be prefix/infix/postfix. – Alexander Jan 19 '20 at 14:24
Yeah I would have just said `SubSequence(self)`. Unclear what `[...]` gains but it's sure weird and cool-looking! Seems like a bug that it's even allowed. – matt Jan 19 '20 at 23:29
@matt or `self[startIndex.. – Leo Dabus Jan 20 '20 at 00:07
The advantage is probably not duplicating unnecessarily exactly the same data already on memory. – Leo Dabus Jan 20 '20 at 00:14
@matt I like it, it's kinda intuitive. It matches the visual style to the return expression below it. Although there's no counterpart for making an empty slice, so it's not all consistent, unfortunately – Alexander Jan 20 '20 at 01:54
To me, your `self[...], SubSequence()` is less intuitive than `SubSequence(self), SubSequence()`. – matt Jan 20 '20 at 05:11

score 1 · Answer 2 · answered Jan 18 '20 at 23:58

1

Nothing built into Swift natively. My best suggestion would be to make an Array extension and do a findIndex passing in the where parameter, then splitting the Array based on that.

answered Jan 18 '20 at 23:58

Charlie Fish

18,491
19
86
179

In Swift, is there a builtin method for splitting an array into a prefix and a suffix?

2 Answers2