How can I initialize Decimal without losing precision in Swift

user_15579458 0
Viewed 195
let decimalA: Decimal = 3.24
let decimalB: Double = 3.24
let decimalC: Decimal = 3.0 + 0.2 + 0.04
print (decimalA)    // Prints 3.240000000000000512
print (decimalB)    // Prints 3.24
print (decimalC)    // Prints 3.24

I'm totally confused. Why do these things happen? I know why floating point numbers lose precision, but I can't understand why Decimal lose precision while storing decimal numbers.

I want to know how can I initialize Decimal type without losing precision. The reason why these happen is also very helpful to me. Sorry for my poor English.

swift decimal
1 Answers

The problem is that all floating point literals are inferred to have type Double, which results in a loss of precision. Unfortunately Swift can't initialise floating point literals to Decimal directly.

If you want to keep precision, you need to initialise Decimal from a String literal rather than a floating point literal.

let decimalA = Decimal(string: "3.24")!
let double = 3.24
let decimalC: Decimal = 3.0 + 0.2 + 0.04
print(decimalA) // Prints 3.24
print(double) // Prints 3.24
print(decimalC) // Prints 3.24

Bear in mind this issue only happens with floating point literals, so if your floating point numbers are generated/parsed in runtime (such as reading from a file or parsing JSON), you shouldn't face the precision loss issue.

user_46678350
Related