NumberFormatter wrong result for 0.9972

I have a double:

let value = 0.99720317490866084

And a Double extention function:

extension Double {
    func stringWithFixedFractionDigits(min: Int, max: Int) -> String {
        let formatter = NumberFormatter()
        formatter.minimumFractionDigits = min
        formatter.maximumFractionDigits = max
        formatter.minimumIntegerDigits = 1
        let numberObject = NSNumber(value: self)
        return formatter.string(from: numberObject) ?? "\(self)"
    }
}

If I use:

value.stringWithFixedFractionDigits(min: 2, max: 2)

I get 1.00 but I would like to get 0.99

What can I change?

You just need to set your NumberFormatter rounding mode property to .down:

formatter.roundingMode = .down

Note that you don't need to create a new NSNumber object, you can safely cast from Double to NSNumber or use string(for: Any) method instead. As an alternative you can extend the protocol FloatingPoint to make it available to all Float types :

extension Formatter {
    static let number = NumberFormatter()
}
extension FloatingPoint {
    func formattedWithFractionDigits(minimum: Int = 2, maximum: Int = 2, minimumIntegerDigits: Int = 1, roundingMode: NumberFormatter.RoundingMode = .halfEven) -> String {
        Formatter.number.roundingMode = roundingMode
        Formatter.number.minimumFractionDigits = minimum
        Formatter.number.maximumFractionDigits = maximum
        Formatter.number.minimumIntegerDigits = minimumIntegerDigits
        return Formatter.number.string(for: self) ?? ""
    }
}


0.9972031749.formattedWithFractionDigits()                                              // 1.00
0.9972031749.formattedWithFractionDigits(roundingMode: .down)                           // "0.99"
0.9972031749.formattedWithFractionDigits(minimumIntegerDigits: 0, roundingMode: .down)  // ".99"