如何定义自定义运算符来处理 Swift 中的 String.Index 范围？

Question

I find using String.Index really needs a lot of code in Swift, especially when it comes to methods of Range Swift doesn't have. Like in above code where I don't want an open range (both bounds exclusive). So I wonder if I can extend String or Range or something to simplify it. In the following code, I already know that startIndex and endIndex are of type String.Index , and startIndex..<endIndex is of type Range<String.Index> . But when I extend String.Index , I'd like to define a method like static func >.< (lhs: String.Index, rhs: String.Index) -> Range<String.Index> , but I failed because there's no method to step String.Index up or down.

let startIndex = str.index(str.firstIndex(of: "[") ?? str.startIndex, offsetBy: 1)
let endIndex = str.firstIndex(of: "]") ?? str.startIndex
let subStr = str[startIndex..<endIndex]

I want to define operators like below. To clarify them using interval notation , if native method a...b is equivalent to [a, b] and a..<b is equivalent to [a, b), what is the equivalent of (a, b) and (a,b].

let startIndex = str.firstIndex(of: "[") ?? str.startIndex
let endIndex = str.firstIndex(of: "]") ?? str.startIndex
let subStr1 = str[startIndex...endIndex]    // [a, b]
let subStr3 = str[startIndex..<endIndex]    // [a, b)
let subStr2 = str[startIndex>.<endIndex]    // (a, b)
let subStr4 = str[startIndex>..endIndex]    // (a, b]

Answer 1

Edit: See the second code example instead; Please disregard the first block, I didn't read the question as carefully as I should have. so it is not a relevant solution.

Edit 2: Also look at the first comment below this answer for an example of a major caveat of the first example. It does not work correctly for strings which contain emojis. Credit to Leo Dabus for this discovery.

---

This code might do what you're looking for, but I can't guarantee its reliability in all cases.

What it does is take a regular half-open range of integers (of the form a..<b ) and a specific string (because, as matt mentioned in his comment, Swift string indices don't pertain to the class String , only to some particular string), and returns an open range of indices for that particular string.

In effect, it just adds 1 to the lower bound to change it from inclusive to exclusive.

import Foundation

// Returns a Range of 'String.Index'es where both upper and lower bounds are exclusive.
extension Range {
    // Here, the upper bound of the range was already exclusive, so don't subtract 1 from it.  Only add 1 to the lower bound.
    static func OpenRange(string: String, range: Range<Int>) -> Range<String.Index> {
        return Range<String.Index>(uncheckedBounds: (String.Index(utf16Offset: range.lowerBound + 1, in: string), String.Index(utf16Offset: range.upperBound, in: string)))
    }
}

let theString = "abcdefg"
let endIndex = theString.index(theString.startIndex, offsetBy: 5)
let range: Range<String.Index> = theString.startIndex..<endIndex

let openRange = Range<String.Index>.OpenRange(string: theString, range: 0..<5)
print(theString[range]) // prints 'abcde'
print(theString[openRange]) // prints 'bcde'

Reference: https://www.avanderlee.com/swift/ranges-explained/

The example I had above doesn't really fit the specific case you were asking about though, as I was starting from code I already had and was trying to 'adapt' it to this situation. My bad: I believe this is a better alternative:

// Returns a Range of 'String.Index'es between two specified characters in the string (as exclusive bounds)
extension Range {
    // Here, the upper bound of the range was already exclusive, so don't subtract 1 from it.  Only add 1 to the lower bound.
    static func OpenRange(string: String, from: String, to: String) -> Range<String.Index> {
        // The 'firstIndex' method expects a Character, not a String, so you need a type cast here
        let lowerInclusive = string.firstIndex(of: Character(from))!
        let lowerExclusive = string.index(lowerInclusive, offsetBy: 1)
        let upperExclusive = string.firstIndex(of: Character(to))!
        return Range<String.Index>(uncheckedBounds: (lowerExclusive, upperExclusive))
    }
}

let theString = "[10:02.11]"
let openRange1 = Range<String.Index>.OpenRange(string: theString, from: "[", to: ":")
let openRange2 = Range<String.Index>.OpenRange(string: theString, from: ":", to: "]")

print(theString[openRange1]) // prints '10'
print(theString[openRange2]) // prints '02.11'