这是一个更灵活的算法舍入到N位有效数字

Swift 3解决方案

extension Double {
// Rounds the double to 'places' significant digits
  func roundTo(places:Int) -> Double {
    guard self != 0.0 else {
        return 0
    }
    let divisor = pow(10.0, Double(places) - ceil(log10(fabs(self))))
    return (self * divisor).rounded() / divisor
  }
}


// Double(0.123456789).roundTo(places: 2) = 0.12
// Double(1.23456789).roundTo(places: 2) = 1.2
// Double(1234.56789).roundTo(places: 2) = 1200

Swift 2扩展

一个更通用的解决方案是以下扩展，适用于Swift 2和iOS 9:

extension Double {
    /// Rounds the double to decimal places value
    func roundToPlaces(places:Int) -> Double {
        let divisor = pow(10.0, Double(places))
        return round(self * divisor) / divisor
    }
}

Swift 3扩展

在Swift 3中，round被圆润取代:

extension Double {
    /// Rounds the double to decimal places value
    func rounded(toPlaces places:Int) -> Double {
        let divisor = pow(10.0, Double(places))
        return (self * divisor).rounded() / divisor
    }
}

返回Double四舍五入到小数点后4位的示例:

let x = Double(0.123456789).roundToPlaces(4)  // x becomes 0.1235 under Swift 2
let x = Double(0.123456789).rounded(toPlaces: 4)  // Swift 3 version

这是一个更灵活的算法舍入到N位有效数字

Swift 3解决方案

extension Double {
// Rounds the double to 'places' significant digits
  func roundTo(places:Int) -> Double {
    guard self != 0.0 else {
        return 0
    }
    let divisor = pow(10.0, Double(places) - ceil(log10(fabs(self))))
    return (self * divisor).rounded() / divisor
  }
}


// Double(0.123456789).roundTo(places: 2) = 0.12
// Double(1.23456789).roundTo(places: 2) = 1.2
// Double(1234.56789).roundTo(places: 2) = 1200

这在Swift 5中似乎有效。

令人惊讶的是，现在还没有这样的标准函数。

//用四舍五入截断双位数到小数点后n位

extension Double {

    func truncate(to places: Int) -> Double {
    return Double(Int((pow(10, Double(places)) * self).rounded())) / pow(10, Double(places))
    }

}

Lots of example are using maths, the problem is floats are approximations of real number, there is no way to express 0.1 (1/10) exactly as a float just as there is no exact way to express ⅓ exactly using decimal points, so you need to ask your self exactly what your are trying to achieve, if you just want to display them leave them as they are in code, trying to round them is going to justify give you less accurate result as you are throwing away precious, round ⅓ in decimal notation to 1 decimal place is not going to give you a number closer to ⅓, us NumberFormate to round it, if you have something like a viewModel class it can be used to return a string representation to your models numbers. NumberFormaters give you lots of control on how numbers are formatted and the number of decimal places you want.

//find the distance between two points
let coordinateSource = CLLocation(latitude: 30.7717625, longitude:76.5741449 )
let coordinateDestination = CLLocation(latitude: 29.9810859, longitude: 76.5663599)
let distanceInMeters = coordinateSource.distance(from: coordinateDestination)
let valueInKms = distanceInMeters/1000
let preciseValueUptoThreeDigit = Double(round(1000*valueInKms)/1000)
self.lblTotalDistance.text = "Distance is : \(preciseValueUptoThreeDigit) kms"