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.

It would be much easier to use an Int index for strings. The reason you need to create a new String.index for each string is because the characters in Swift are not all the same length under the hood. A single Swift character can consist of one, two or even more Unicode code points. Therefore, each unique string must calculate the indices of its characters.

It is possible to hide this complexity behind an Int index extension, but I hesitate to do so. It’s good to be reminded of what actually happens.

Translated with www.DeepL.com/Translator (free version)

The following Extenson must be added under your class in the code. This extension offers you the possibility to determine the index of a whole string within a string. In my example “ground”.

String operations are thus addressed via indices and ranges. The index is therefore not a simple integer variable.

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
    }
}

Swift Substring to String Conversion