Count number of decimal places in a Float (or Decimal) in Swift

Question

I want to count the number of decimal places (ignoring trailing zeros) in a Float (or NSDecimalNumber) for example:

1.45000 => 2
5.98 => 2
1.00 => 0
0.8         


        

Answer 1

Doing this with Decimal is fairly straightforward, provided you correctly create your Decimal. Decimals are stored as significand * 10^exponent. significand is normalized to the smallest integer possible. So for 1230, the significand is 123 and the exponent is 1. For 1.23 the significand is also 123 and the exponent is -2. That leads us to:

extension Decimal {
    var significantFractionalDecimalDigits: Int {
        return max(-exponent, 0)
    }
}

However, you must be very careful constructing your Decimal. If you construct it from a Double, you will already have applied binary rounding errors. So for example:

let n = Decimal(0.111) // 0.11100000000000002048 because you passed a Double
significantFractionalDecimalDigits // 20

vs.

let n = Decimal(string: "0.111")!
n.significantFractionalDecimalDigits // 3 what you meant

Keep in mind of course that Decimal has a maximum number of significant digits, so it may still apply rounding.

let n = Decimal(string: "12345678901235678901234567890.1234567890123456789")!
n.significantFractionalDecimalDigits // 9 ("should" be 19)

And if you're walking down this road at all, you really must read the Floating Point Guide and the canonical StackOverflow question: Is floating point math broken?