NSDecimalNumber usage for precision with currency in Swift

I've read a lot that NSDecimalNumber is the best format to use when using currency.

However, I'm still getting floating point issues.

For example.

let a: NSDecimalNumber = 0.07 //0.07000000000000003
let b: NSDecimalNumber = 7.dividing(by: 100) //0.06999999999999999

I know I could use Decimal and b would be what I'm expecting:

let b: Decimal = 7 / 100 //0.07

I'm using Core Data in my app. So I'm stuck with NSDecimalNumber. Unless I want convert a lot of NSDecimalNumbers to Decimals.

Can someone help me get 0.07?

Solution

The problem is that you’re effectively doing floating point math (with the problems it has faithfully capturing fractional decimal values in a Double) and creating a Decimal (or NSDecimalNumber) from the Double value that already has introduced this discrepancy. Instead, you want to create your Decimal values before doing your division (or before having a fractional Double value, even if a literal).

So, the following is equivalent to your example, whereby it is building a Double representation (with the limitations that entails) of 0.07, and you end up with a value that is not exactly 0.07:

let value = Decimal(7.0 / 100.0)                                // or NSDecimalNumber(value: 7.0 / 100.0)

Whereas this does not suffer this problem because we are dividing a decimal 7 by a decimal 100:

let value = Decimal(7) / Decimal(100)                           // or NSDecimalNumber(value: 7).dividing(by: 100)

Or, other ways to create 0.07 value but avoiding Double in the process include using strings:

let value = Decimal(string: "0.07")                             // or NSDecimalNumber(string: "0.07")

Or specifying the mantissa/significant and exponent:

let value = Decimal(sign: .plus, exponent: -2, significand: 7)  // or NSDecimalNumber(mantissa: 7, exponent: -2, isNegative: false)

Bottom line, avoid Double representations entirely when using Decimal (or NSDecimalNumber), and you won't suffer the problem you described.