[String To Integer]

String+Extension.swift

import Foundation

precedencegroup ExponentiationPrecedence {
    associativity: right
    higherThan: MultiplicationPrecedence
}

infix operator **: ExponentiationPrecedence

func ** (lhs: Int, rhs: Int) -> Int {
    return Int(pow(Float(lhs), Float(rhs)))
}

var sampleString = "3459"

extension String {
    func convert() -> Int? {
        var result: Int = 0

        // building value map
        var valueMap = [
            "0" as Character: 0,
            "1": 1,
            "2": 2,
            "3": 3,
            "4": 4,
            "5": 5,
            "6": 6,
            "7": 7,
            "8": 8,
            "9": 9
        ]

        // example: 3459
        // 3459 = (3 * 10^3) + 4 * 10^2 + 5 * 10^1 + 9 * 10^0
        //         value * (10 ** exponent)
        for (i, c) in self.enumerated() {
            let exponent = self.count - i - 1
            if let value = valueMap[c] {
                result += value * (10 ** exponent)
            } else {
                return nil
            }
        }
        return result
    }
}

sampleString.convert() // 3459