在Swift编程语言中获取字符串的第n个字符

注意： 请参阅LeoDabus的答案以获取有关Swift 4和Swift5的正确实现。

Xpre 11•Swift 5.1

您可以扩展StringProtocol以使下标也可用于子字符串：

extension StringProtocol {    subscript(_ offset: Int)          -> Element     { self[index(startIndex, offsetBy: offset)] }    subscript(_ range: Range<Int>)    -> SubSequence { prefix(range.lowerBound+range.count).suffix(range.count) }    subscript(_ range: ClosedRange<Int>)         -> SubSequence { prefix(range.lowerBound+range.count).suffix(range.count) }    subscript(_ range: PartialRangeThrough<Int>) -> SubSequence { prefix(range.upperBound.advanced(by: 1)) }    subscript(_ range: PartialRangeUpTo<Int>)    -> SubSequence { prefix(range.upperBound) }    subscript(_ range: PartialRangeFrom<Int>)    -> SubSequence { suffix(Swift.max(0, count-range.lowerBound)) }}extension LosslessStringConvertible {    var string: String { .init(self) }}extension BidirectionalCollection {    subscript(safe offset: Int) -> Element? {        guard !isEmpty, let i = index(startIndex, offsetBy: offset, limitedBy: index(before: endIndex)) else { return nil }        return self[i]    }}

测试中

let test = "Hello USA