Which Swift datatype do I use for currency

Question

I have an iOS application that will be performing a lot of basic arithmetic on numbers representing USD currency (eg 25.00 representing $25.00).

I have gotten into a lot of trouble using the datatype Double in other languages like Java and Javascript so I would like to know the best datatype to use for currency in Swift.

Answer 1

Use Decimal, and make sure you initialize it properly!

CORRECT

// Initialising a Decimal from a Double:
let monetaryAmountAsDouble = 32.111
let decimal: Decimal = NSNumber(floatLiteral: 32.111).decimalValue
print(decimal) // 32.111  
let result = decimal / 2
print(result) // 16.0555 


// Initialising a Decimal from a String:
let monetaryAmountAsString = "32,111.01"

let formatter = NumberFormatter()
formatter.locale = Locale(identifier: "en_US")
formatter.numberStyle = .decimal

if let number = formatter.number(from: monetaryAmountAsString) {
    let decimal = number.decimalValue
    print(decimal) // 32111.01 
    let result = decimal / 2.1
    print(result) // 15290.9571428571428571428571428571428571 
}

INCORRECT

let monetaryAmountAsDouble = 32.111
let decimal = Decimal(monetaryAmountAsDouble) 
print(decimal) // 32.11099999999999488  

let monetaryAmountAsString = "32,111.01"
if let decimal = Decimal(string: monetaryAmountAsString, locale: Locale(identifier: "en_US")) {
    print(decimal) // 32  
}

Performing arithmetic operations on Doubles or Floats representing currency amounts will produce inaccurate results. This is because the Double and Float types cannot accurately represent most decimal numbers. More information here.

Bottom line: Perform arithmetic operations on currency amounts using Decimals or Int

Answer 2

I suggest you start with a typealias for Decimal. Example:

typealias Dollars = Decimal
let a = Dollars(123456)
let b = Dollars(1000)
let c = a / b
print(c)

Output:

123.456

If you are trying to parse a monetary value from a string, use a NumberFormatter and set its generatesDecimalNumbers property to true.

Answer 3

There's a really nice lib called Money:

let money: Money = 100
let moreMoney = money + 50 //150

There a lot of nice features besides that, such as type-safe currencies:

let euros: EUR = 100
let dollars: USD = 1500
euros + dollars //Error

Binary operator '+' cannot be applied to operands of type 'EUR' (aka '_Money') and 'USD' (aka '_Money')

Answer 4

The Flight-School/Money is probably the best choice for a project full of monies.

In the ending of README @mattt provides a nice solution for a simple Money type:

struct Money {
    enum Currency: String {
      case USD, EUR, GBP, CNY // supported currencies here
    }

    var amount: Decimal
    var currency: Currency
}

Chapter 3 in 'Flight School Guide to Swift Numbers' is provides excellent intro into the topic.

Answer 5

We use this approach with Currency being a huge enum that has been generated in advance (don't forget a monetary value is just a number without its currency - they both belong together):

struct Money: Hashable, Equatable {

    let value: Decimal
    let currency: Currency
}

extension Money {

    var string: String {
        CurrencyFormatter.shared.string(from: self)
    }
}

Answer 6

You need the Decimal and its initializer:

Decimal(string: "12345.67890", locale: Locale(identifier: "en-US"))

When considering the other answers, please note that Float and Double literals affect the accuracy of the Decimal result. The same applies to the NumberFormatter.

Decimal(5.01) // 5.009999999999998976 ❌

Decimal(floatLiteral: 5.01) // 5.009999999999998976 ❌

NSNumber(floatLiteral: 0.9999999).decimalValue // 0.9999999000000001 ❌

let decimalFormatter = NumberFormatter()
decimalFormatter.numberStyle = .decimal
decimalFormatter.number(from: "0.9999999")?.decimalValue // 0.9999999000000001 ❌

Decimal(string: "5.01", locale: Locale(identifier: "en-US")) // 5.01 ✅
Decimal(string: "0.9999999", locale: Locale(identifier: "en-US")) // 0.9999999 ✅