swift - 如何计算出一个字符串中子字符串的范围，然后在另一个字符串中使用它

Question

基本上我想要这样的东西，

NSString* foobar(NSString *input) {
    // say input is "1"
    NSString *string = @"0123456789";
    NSString *anotherString = @"零一二三四五六七八九";
    
    NSRange range = [string rangeOfString:input];
    
    // return "一" here
    return [anotherString substringWithRange:range];
}

我在 Swift 中尝试了同样的东西，

func foobar(input: String) -> String {
    // say input is "1"
    let string = "0123456789"
    
    let range = string.range(of: input, options: .anchored)
    let result = anotherString[range!]
    
    // return "012" here
    return String(result)
}

为什么？我怎样才能实现这一目标？

Answer 1

字符串(或通常是集合)索引只能与创建它们的集合一起使用。为了在另一个字符串中找到相同的位置，索引必须转换为(整数)偏移量并返回到目标字符串的索引:

func foobar(input: String) -> String? {
    let s1 = "0123456789"
    let s2 = "😀一二三四五六七八九";

    guard let range = s1.range(of: input) else {
        return nil
    }
    let pos = s1.distance(from: s1.startIndex, to: range.lowerBound)
    let len = s1.distance(from: range.lowerBound, to: range.upperBound)
    guard
        let lo = s2.index(s2.startIndex, offsetBy: pos, limitedBy: s2.endIndex),
        let hi = s2.index(lo, offsetBy: len, limitedBy: s2.endIndex)
    else {
        return nil
    }
    return String(s2[lo..<hi])
}

print(foobar(input: "1") as Any) // Optional("一")
print(foobar(input: "123") as Any) // Optional("一二三")
print(foobar(input: "124") as Any) // nil

只要字符串中的所有字符都使用单个 UTF-16 代码单元(因为这是 NSRange 所计数的)，您的 Objective-C 代码就可以工作。它无法正确处理表情符号、标志和其他表示为 UTF-16 代理项对的字符，例如与

NSString *anotherString = @"😀一二三四五六七八九";