generatesDecimalNumbers for NumberFormatter does not work

问题

My function is converting a string to Decimal

func getDecimalFromString(_ strValue: String) -> NSDecimalNumber {
    let formatter = NumberFormatter()
    formatter.maximumFractionDigits = 1
    formatter.generatesDecimalNumbers = true
    return formatter.number(from: strValue) as? NSDecimalNumber ?? 0
}

But it is not working as per expectation. Sometimes it's returning like

Optional(8.300000000000001)
Optional(8.199999999999999)

instead of 8.3 or 8.2. In the string, I have value like "8.3" or "8.2" but the converted decimal is not as per my requirements. Any suggestion where I made mistake?

回答1:

That seems to be a bug, compare

NSDecimalNumbers from NSNumberFormatter are affected by binary approximation error

Even with generatesDecimalNumbers set to true, the formatter produces a binary floating point number internally, and that cannot represent decimal fractions like 8.2 precisely.

Note also that (contrary to the documentation), the maximumFractionDigits property has no effect when parsing a string into a number.

There is a simple solution: Use

NSDecimalNumber(string: strValue) // or
NSDecimalNumber(string: strValue, locale: Locale.current)

instead, depending on whether the string is localized or not.

Or with the Swift 3 Decimal type:

Decimal(string: strValue) // or
Decimal(string: strValue, locale: .current)

Example:

if let d = Decimal(string: "8.2") {
    print(d) // 8.2
}

来源：https://stackoverflow.com/questions/43538661/generatesdecimalnumbers-for-numberformatter-does-not-work

标签

swift

numbers

decimal