在 Swift 中一次迭代两个集合

假设我有一个数组[1, 2, 3, 4, 5]。我怎样才能一次迭代两个？

Iteration 1: (1, 2)
Iteration 2: (3, 4)
Iteration 3: (5, nil)

您可以使用名为 stride(to:, by:) 的渐进循环来每隔 n 个元素迭代元素：

let array = Array(1...5)

let pairs = stride(from: 0, to: array.endIndex, by: 2).map {
    (array[$0], $0 < array.index(before: array.endIndex) ? array[$0.advanced(by: 1)] : nil)
}   // [(.0 1, {some 2}), (.0 3, {some 4}), (.0 5, nil)]

print(pairs)  // "[(1, Optional(2)), (3, Optional(4)), (5, nil)]\n"

要迭代集合子序列而不是元组：

extension Collection {
    func unfoldSubSequences(limitedTo maxLength: Int) -> UnfoldSequence<SubSequence,Index> {
        sequence(state: startIndex) { start in
            guard start < self.endIndex else { return nil }
            let end = self.index(start, offsetBy: maxLength, limitedBy: self.endIndex) ?? self.endIndex
            defer { start = end }
            return self[start..<end]
        }
    }
}

let array = Array(1...5)
for subsequence in array.unfoldSubSequences(limitedTo: 2) {
    print(subsequence)  // [1, 2] [3, 4] [5]
}

这适用于任何类型的集合：

let string = "12345"
for substring in string.unfoldSubSequences(limitedTo: 2) {
    print(substring)  // "12" "34" "5"
}