逐字循环数组

CycleSequence Collection Sequence n 元素与 .prefix(n)

struct CycleSequence<C: Collection>: Sequence {
    let cycledElements: C

    init(cycling cycledElements: C) {
        self.cycledElements = cycledElements
    }

    public func makeIterator() -> CycleIterator<C> {
        return CycleIterator(cycling: cycledElements)
    }
}

struct CycleIterator<C: Collection>: IteratorProtocol {
    let cycledElements: C
    var cycledElementIterator: C.Iterator

    init(cycling cycledElements: C) {
        self.cycledElements = cycledElements
        self.cycledElementIterator = cycledElements.makeIterator()
    }

    public mutating func next() -> C.Iterator.Element? {
        if let next = cycledElementIterator.next() {
            return next
        } else {
            self.cycledElementIterator = cycledElements.makeIterator() // Cycle back again
            return cycledElementIterator.next()
        }
    }
}

print(Array(CycleSequence(cycling: [true, false]).prefix(7)))
print(Array(CycleSequence(cycling: 1...3).prefix(7)))
print(Array(CycleSequence(cycling: "ABC").prefix(7)))
print(Array(CycleSequence(cycling: EmptyCollection<Int>()).prefix(7)))
print(Array(zip(1...10, CycleSequence(cycling: "ABC"))))

输出:

[true, false, true, false, true, false, true]
[1, 2, 3, 1, 2, 3, 1]
["A", "B", "C", "A", "B", "C", "A"]
[]
[(1, "A"), (2, "B"), (3, "C"), (4, "A"), (5, "B"), (6, "C"), (7, "A"), (8, "B"), (9, "C"), (10, "A")]

模算子 % 会是你在这里的朋友。 我没有一个编译器在我面前,总是搞砸范围语法,但下面应该说明这个想法

for i in 0..<100 {
    let theItem = array[i % array.count]
}

有外遇 for forEach 循环遍历数组。

let array = [Int]()

for index in 0..<100 {
    array.forEach { element in
        print(element)
    }
}

简而言之: 在迭代中迭代一百次。