在Swift编程语言中获取字符串的第n个字符

问题描述

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

Xcode 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 
解决方法

                    
如何获得字符串的第n个字符？我尝试[]没有成功的bracket（）访问器。
var string = "Hello,world!"

var firstChar = string[0] // Throws error


错误：“下标”不可用：无法对带Int的字符串进行下标，请参见文档注释以进行讨论