swift - How to store 1.66 in NSDecimalNumber

Question

I know float or double are not good for storing decimal number like money and quantity. I'm trying to use NSDecimalNumber instead. Here is my code in Swift playground.

let number:NSDecimalNumber = 1.66
let text:String = String(describing: number)
NSLog(text)

The console output is 1.6599999999999995904

How can I store the exact value of the decimal number 1.66 in a variable?

Answer 1

In

let number:NSDecimalNumber = 1.66

the right-hand side is a floating point number which cannot represent the value "1.66" exactly. One option is to create the decimal number from a string:

let number = NSDecimalNumber(string: "1.66")
print(number) // 1.66

Another option is to use arithmetic:

let number = NSDecimalNumber(value: 166).dividing(by: 100)
print(number) // 1.66

With Swift 3 you may consider to use the "overlay value type" Decimal instead, e.g.

let num = Decimal(166)/Decimal(100)
print(num) // 1.66

Yet another option:

let num = Decimal(sign: .plus, exponent: -2, significand: 166)
print(num) // 1.66

Addendum:

Related discussions in the Swift forum:

• Exact NSDecimalNumber via literal
• ExpressibleByFractionLiteral

Related bug reports:

• SR-3317 Literal protocol for decimal literals should support precise decimal accuracy, closed as a duplicate of
• SR-920 Re-design builtin compiler protocols for literal convertible types.