How to format Decimal in Swift 3

Question

I'm trying to use the Swift Decimal Structure for currency operations but I cannot format it.

How can I format var myDecimal:Decimal = 9999.99 to display $9,999.99?

Without using Decimals I can do it as follow...

let myTotal:NSNumber = 9999.99

let currencyFormatter = NumberFormatter()
currencyFormatter.usesGroupingSeparator = true

currencyFormatter.numberStyle = .currency
currencyFormatter.locale = NSLocale.current
let priceString = currencyFormatter.string(from: myTotal)

myLabel.text = priceString

This works fine but I have been reading and Decimalseem to be the right type for currency.

I tried...

let myTotal:Decimal = 9999.99

let currencyFormatter = NumberFormatter()
currencyFormatter.usesGroupingSeparator = true

currencyFormatter.numberStyle = .currency
// localize to your grouping and decimal separator
currencyFormatter.locale = NSLocale.current
let priceString = currencyFormatter.string(from: NSNumber(myTotal))

myLabel.text = priceString

... but I get error

Argument labels '(_:)' do not match any available overloads

What is the right way to format Decimals in Swift?

Poorhouse

Just use string(for: myTotal) instead of string(from: – Ahmednagar

Check this #29783482 – Ahmednagar

I don't think you'll see any benefit from using Decimal over Float or Double unless you need high precision, which is unlikely for currency. – Bioplasm

@LeoDabus string(for: myTotal) did the trick. Thanks. – Poorhouse

@DaveWood - What would you considered high precision, when would you use Decimal? – Poorhouse

@Poorhouse High precision is needed for things like 3D graphics, where you can have lots of digits after the decimal point. If you're only using 2 decimal places, you can avoid complicating your code and just use Floats. – Bioplasm

@Poorhouse you are welcome. Note that the currencyField of the linked question can be used with any currency including the ones that doesn't have any fraction digits at all. – Ahmednagar

Never use float or double for currency. See #3730519 – Cognoscenti

Answer 1

You can just cast your Decimal to an NSDecimalNumber first:

let priceString = currencyFormatter.string(from: myTotal as NSDecimalNumber)

Wichman

string(from:) is more specific, since it takes a number type as an argument rather than Any. – Wichman

NSDecimalNumber is an NSNumber type. – Wichman

string(for:) is a method on the abstract Formatter superclass. It is a nonspecific API that applies to any Formatter subclass, and the documentation says nothing about what it does when used on a NumberFormatter instance. string(from:) is actually mentioned in the NumberFormatter documentation, and what it does is actually documented. Therefore, I prefer it. In practice, it probably doesn't strictly matter since in all likelihood, one of these methods probably just calls through to the other. – Wichman

They used Any because the method is on the abstract superclass, Formatter, and thus has no idea what types of objects it's going to take, or even whether it's a number formatter, date formatter, a person name formatter, some crazy custom formatter, or something else. They actually did add a method for each type Double, Float, Decimal, in a sense, because they added a method for NSNumber, which is the Objective-C type that all those types bridge to. – Wichman