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.