ByTobias Stephan

Swift Substring to String Conversion

These string operations in Swift can drive you crazy if you are used to a simple one from C#. So, I’ve been looking into it and I’ve put together a little bit here. I tested it with Swift 5.1

• startIndex is the index of the first character
• endIndex is the index after the last character

 override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        let str = "Hello, playground"
        print(str[str.startIndex]) // H
        //print(str[str.endIndex]) // error: after last character

        let rangeStartToEnd = str.startIndex..<str.endIndex
        print("Range: " + str [rangeStartToEnd]) // "Hello, playground"

        let rangeWithoutGround = str.startIndex..<str.index(of: "ground")!
        print("Range: " + str[rangeWithoutGround]) // "Hello, playground"
    }

before refers to the index of the character directly before the specified index.

// character
let index = str.index(before: str.endIndex)
str[index] // d

// range
let range = str.startIndex..<str.index(before: str.endIndex)
str[range] // Hello, playgroun

The OffsetBy value may be positive or negative and starts from the specified index. Although it is of type String.IndexDistance, you can pass an Int value.

// character
let index = str.index(str.startIndex, offsetBy: 7)
str[index] // p

// range
let start = str.index(str.startIndex, offsetBy: 7)
let end = str.index(str.endIndex, offsetBy: -6)
let range = start..<end
str[range] // play

The limitedBy is useful to ensure that the offset does not cause the index to exceed the limits. It is a limit index. Since it is possible for the offset to exceed the limit, this method returns an optional. It returns zero if the index is outside the limits.

if let index = str.index(str.startIndex, offsetBy: 7, limitedBy: str.endIndex) {
    str[index] // p
}

If the offset had been 77 instead of 7, the if statement would have been skipped.

extension StringProtocol {
    func index<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> Index? {
        range(of: string, options: options)?.lowerBound
    }
    func endIndex<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> Index? {
        range(of: string, options: options)?.upperBound
    }
    func indices<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> [Index] {
        var indices: [Index] = []
        var startIndex = self.startIndex
        while startIndex < endIndex,
            let range = self[startIndex...]
                .range(of: string, options: options) {
                indices.append(range.lowerBound)
                startIndex = range.lowerBound < range.upperBound ? range.upperBound :
                    index(range.lowerBound, offsetBy: 1, limitedBy: endIndex) ?? endIndex
        }
        return indices
    }
    func ranges<S: StringProtocol>(of string: S, options: String.CompareOptions = []) -> [Range<Index>] {
        var result: [Range<Index>] = []
        var startIndex = self.startIndex
        while startIndex < endIndex,
            let range = self[startIndex...]
                .range(of: string, options: options) {
                result.append(range)
                startIndex = range.lowerBound < range.upperBound ? range.upperBound :
                    index(range.lowerBound, offsetBy: 1, limitedBy: endIndex) ?? endIndex
        }
        return result
    }
}