问题
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?
回答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.
来源：https://stackoverflow.com/questions/42781785/how-to-store-1-66-in-nsdecimalnumber