How to round Double values?

Question

I made a function to return the rounding of values of type Double, the function is working, but I do not know if it is the ideal way to have this result, I think there must be a way without having to use the class String.

var numero: Double = 6.73865

func arredonda(valor: Double, casasdecimais: Int)-> Double{
    let formato = String(casasdecimais)+"f"
    return Double(String(format: "%."+formato, valor))!
}

arredonda(numero, casasdecimais:  3)  

// returns 6.739

arredonda(numero, casasdecimais:  2) 

// returns 6.74

Answer 1

There’s no way to have control over the decimal places in a number like double, this is characteristic of it. If you need this, you need to use another data type. In this case it would be the NSDecimalNumber. The class will also be useful NSNumberFormatter

Read more about this on response on Objective C. And keep following them all links to understand why this occurs.

Behold What is the correct way to use the float, double and decimal types?.

Answer 2

In Swift we can create Extensions (which is similar to the Categories of Objective-C), so we can add a new functionality to the class Double:

extension Double {
    /// Arredonda um Double conforme quantidade de casas decimais
    func arredonda(casas: Int) -> Double {
        let divisor = pow(10.0, Double(casas))
        return round(self * divisor) / divisor
    }
}


let a = Double(0.123456789).arredonda(4)  //0.1235
let b = Double(0.123456789).arredonda(5)  //0.12346
let c = Double(6.73865).arredonda(3)  //6.739
let d = Double(6.73865).arredonda(2)  //6.74

Answer 3

After the replies of @Maniero and @iTSanguar I made a function that uses part of the concepts presented by them and also me for this that I found in ONLY

extension Double {
        /// Arredonda um Double conforme quantidade de casas decimais
        func arredonda(casasDecimais: Int) -> Double {
            var formatacao:String {
                let formatacao = NSNumberFormatter()
                formatacao.minimumFractionDigits = casasDecimais
                return formatacao.stringFromNumber(self)!

        }
        return Double(formatacao)!

    }
}
var numero: Double  = 6.73865
numero.arredonda(3) // 6.739
numero.arredonda(2) // 6.74