str[i], str[i..<j], str[i...j]

str[i]

👉 replit

extension String {

    /// ⭐ example: `str[5]`
    public subscript(_ i: Int) -> String? {

        // ⭐ guard: non-empty string, non-negative index 
        guard !isEmpty, i >= 0 else { return nil }

        // ⭐ safe bound
        let bound = index(before: endIndex)     // ⭐ Note: `endIndex` is NOT safe

        // ⭐ guard: startIndex <= index < endIndex
        guard let index = index(startIndex, offsetBy: i, limitedBy: bound) else {
            return nil
        }

        // ⭐ Character -> String
        return String(self[index])
    }

    /// example: `str[2..<5]`
    public subscript(bounds: Range<Int>) -> String? {

        // ⭐ guard: empty string
        guard !isEmpty else { return nil }      // empty string -> nil

        // ⭐ guard: 0 <= m < n
        let (m, n) = (bounds.lowerBound, bounds.upperBound)
        guard 0 <= m, m < n else { return nil }        // negative index -> nil

        // ⭐ guard: i < endIndex, j <= endIndex
        let bound = index(before: endIndex)
        guard let i = index(startIndex, offsetBy: m, limitedBy: bound) else {
            return nil
        }
        guard let j = index(startIndex, offsetBy: n, limitedBy: endIndex) else {
            return nil
        }

        // i, j are in safe range
        return String(self[i..<j])
    }

    /// example: `str[2...5]`
    public subscript(bounds: ClosedRange<Int>) -> String? {

        // ⭐ guard: empty string
        guard !isEmpty else { return nil }      // empty string -> nil

        // ⭐ guard: 0 <= m < n
        let (m, n) = (bounds.lowerBound, bounds.upperBound)
        guard 0 <= m, m <= n else { return nil }        // negative index -> nil

        // ⭐ guard: i < endIndex, j <= endIndex
        let bound = index(before: endIndex)
        guard let i = index(startIndex, offsetBy: m, limitedBy: bound) else {
            return nil
        }
        guard let j = index(startIndex, offsetBy: n, limitedBy: bound) else {
            return nil
        }

        // i, j are in safe range
        return String(self[i...j])
    }
}

💈範例

👉 replit

//       01234
let s = "Swift"
s[1]    // String?: "w"
s[5]    // nil

// half-open range
s[0..<3],               // Optional("Swi")
s[2..<5],               // Optional("ift")
s[1..<1],               // nil
s[1..<9],               // nil
s[-3..<2],              // nil

// closed range
s[0...3],               // Optional("Swif")
s[2...5],               // nil
s[1...1],               // Optional("w")
s[1...9],               // nil
s[-3...2],              // nil

👥 相關

• String.Index

• Subscripts

📘 手冊

• Swift ⟩

  • Subscripts

  • Collections ⟩

    • Range<Bound> - (generic struct)

    • BidirectionalCollection ⟩

      • subscript(_:) - arr[i..<j], str[i..<j] 都是 override 這個方法

🗣 討論

• 'subscript' is unavailable: cannot subscript String with a CountableClosedRange