How to take a Roman Numeral String and convert it to base10 digit?

Question

So I am building a sample project which takes roman numerals and converts it to a base10 digit. I tried getting this to work but have not accomplished my goal. Is there anyone who knows how to accomplish this? I am adding the roman numerals up to 10 below for reference. Thanks!

1 - I 2 - II 3 - III 4 - IV 5 - V 6 - VI 7 - VII 8 - VIII 9 - IX 10 - X

Answer 1

All you need is to iterate through your roman string characters in a reverse way and map those characters values. Start with maxValue equal to zero, switch the letters value, save it as the maxValue and if the value is equal to maxValue add it otherwise subtract the actual value. You can also use regex (strict or not) to validate and throw an error in case it fails. Try like this:

Xcode 9.x • Swift 4.x

Note: for Swift 3 version or earlier check the edit history.

extension String {
    enum RomanParsingError: Error {
        case invalidNumber
    }
    func romanNumeralValue() throws -> Int  {
        guard range(of: "^(?=[MDCLXVI])M*(C[MD]|D?C{0,3})(X[CL]|L?X{0,3})(I[XV]|V?I{0,3})$", options: .regularExpression) != nil else {
            throw RomanParsingError.invalidNumber
        }
        var result = 0
        var maxValue = 0
        uppercased().reversed().forEach {
            let value: Int
            switch $0 {
            case "M":
                value = 1000
            case "D":
                value = 500
            case "C":
                value = 100
            case "L":
                value = 50
            case "X":
                value = 10
            case "V":
                value = 5
            case "I":
                value = 1
            default:
                value = 0
            }
            maxValue = max(value, maxValue)
            result += value == maxValue ? value : -value
        }
        return result
    }
}

usage:

do {
    let decimal = try "MCMLXXVIII".romanNumeralValue()
    print(decimal)   // 1978
} catch {
    print(error)
}


do {
    let decimal = try "IIIV".romanNumeralValue()
    print(decimal)
} catch {
    print(error)   // "invalidNumber\n"
}

Answer 2

Roman numerals do not have a one-to-one mapping to decimal digits. You mapped out the sequences of roman numerals from 1 to 9 (single decimal digits.) There are also roman numerals for 10, 50, 100, 500, and 1000.

If you want to convert a large number in roman numerals to decimal, you need to parse it. You'll need to handle those larger units first.

Map out on paper how you do the conversion yourself, in small steps. Then write those steps out in pseudocode. (Don't worry about syntax, just map out the steps.)

Finally, use your pseudocode as a guide to writing your Swift program.