Ways to index String with an Int.

StringIndicesPlaygroundContent.swift

// How to Index String by Int
// Andrew Eades

// https://www.youtube.com/channel/UC2kOJKUGXfC01YEKH4QjFRA

// Paste this into an Xcode Playground

import Cocoa

var str = "Hello, playground"

str[str.index(str.startIndex, offsetBy: 7)]
str[str.index(str.startIndex, offsetBy: 7)..<str.index(str.startIndex, offsetBy: 11)]


struct IndexableByInteger<C: RangeReplaceableCollection> {
    var collection: C

    init(_ collection: C) {
        self.collection = collection
    }
    
    private func _index(for n: Int) -> C.Index? {
        return collection.indices.dropFirst(n).first
    }
    
    subscript(intIndex: Int) -> C.Element? {
        get {
            guard let index = _index(for: intIndex)
            else {
                return nil
            }
            return collection[index]
        }
        set {
            guard let index = _index(for: intIndex)
            else {
                return
            }
            
            var sub = C()
            if let newValue = newValue {
                sub.append(newValue)
            }
            collection.replaceSubrange(index...index, with: sub)
        }
    }
    
    subscript(intRange: Range<Int>) -> C.SubSequence? {
        get {
            self[intRange.lowerBound ... intRange.upperBound - 1]
        }
        
        set {
            self[intRange.lowerBound ... intRange.upperBound - 1] = newValue
        }
    }
    
    subscript(intRange: ClosedRange<Int>) -> C.SubSequence? {
        get {
            guard let start = _index(for: intRange.lowerBound),
                  let end = _index(for: intRange.upperBound)
            else {
                return nil
            }
            return collection[start ... end]
        }
        
        set {
            guard let start = _index(for: intRange.lowerBound),
                  let end = _index(for: intRange.upperBound)
            else {
                return
            }
            
            collection.replaceSubrange(start ... end, with: newValue ?? C()[...])
        }
    }
}


extension IndexableByInteger: CustomStringConvertible where C: CustomStringConvertible {
    var description: String {
        collection.description
    }
}

var string = IndexableByInteger(str)

string                          // "Hello, plaground"
string[7]                       // "p"
string[7..<11]                  // "play"

string[0] = "Y"                 // "Y"
string                          // "Yello, playground"
string[0..<5] = "Goodbye"       // "Goodbye"
string                          // "Goodbye, playground"

Removed redundant line.