How to solve iOS error with locale decimal point / comma in calculations? (with NumberFormatter)

Question

I have the following problem with iOS calculations when the user is in different locales. Here in Europe the decimal point separator is a "," and in other locales it might be a ".". As long as the user is in a locale with a "." all calculations work fine, but when a comma is used instead I get an error. Here's the simple code so far:

if let expense = expenseTextField.text
        let expenseValue = Double(expense)
        let guests = guestTextField.text
        let guestValue = Double(guests) {

        let result = round((expenseValue / guestValue) * 100) / 100

        resultLabel.text = priceString
        //resultLabel.text = String(result)
    } else {
        resultLabel.text = "No values!"
    }

I am pretty sure I have to use the numberFormatter as far as I understand other questions on here, but I don't get how to do this and the documentation doesn't provide a sample how to create a double or float from a string. I tried the following but I get an error stating that I cannot neither create doubles nor use these variables for calculations:

 if let expense = expenseTextField.text
        let expenseValue = String(expense),
        let guests = guestTextField.text
        let guestValue = String(guests) {


        let expenseFormatter = NumberFormatter()
        expenseFormatter.number(from: expenseValue)

        let guestFormatter = NumberFormatter()
        guestFormatter.number(from: guestValue)

        let expenseDouble = Double(expenseFormatter)
        let guestDouble = Double(guestFormatter)


        let result = round((expenseDouble / guestDouble) * 100) / 100

        resultLabel.text = priceString
    } else {
        resultLabel.text = "No values!"
    }

I am not sure how to create real Doubles from those strings with numberFormatter and then use them in calculations.

Can someone give me a tip? Thanks

Answer 1

The only thing you need is to write the method call correctly:

let expenseDouble = expenseFormatter.number(from: expenseValue)?.doubleValue ?? 0
let guestDouble = guestFormatter.number(from: guestValue)?.doubleValue ?? 0

A formatter/parser directly converts string to number. You then have to take the Double value from it. Note that parsing can fail and you have to solve the resulting optional in that case, e.g. using ?? 0.