Do you feel like you've learned something and forgotten it?

extension Int {
    func repetitions(task: () -> Void) {
        for _ in 0..<self {
            task()
        }
    }
    mutating func square() {
        self = self * self
    }
    
    subscript(digitIndex: Int) -> Int {
        var decimalBase = 1
        for _ in 0..<digitIndex {
            decimalBase *= 10
        }
        
        return (self / decimalBase) % 10
    }
    
    enum Kind {
        case negative, positive, zero
    }
    
    var kind: Kind {
        switch self {
        case 0:
            return .zero
        case let x where x > 0:
            return .positive
        default:
            return .negative
        }
    }
}

var i = 0
9.repetitions {
    print(987239847[i])
    i += 1
}

var someInt = 3
someInt.square()

func printIntegerKinds(_ numbers: [Int]) {
    for number in numbers {
        switch number.kind {
        case .negative:
            print("- ", terminator: "")
        case .zero:
            print("0 ", terminator: "")
        case .positive:
            print("+ ", terminator: "")
        }
    }
    print("")
}

printIntegerKinds([3, 19, -27, 0, -6, 0, 7])

Excerpt From: Apple Inc. “The Swift Programming Language (Swift 5.3).” Apple Books. https://books.apple.com/us/book/the-swift-programming-language-swift-5-3/id881256329