这是一种长期的变通方法，如果您的需求稍微复杂一点，它可能会派上用场。你可以在Swift中使用数字格式化器。

let numberFormatter: NSNumberFormatter = {
    let nf = NSNumberFormatter()
    nf.numberStyle = .DecimalStyle
    nf.minimumFractionDigits = 0
    nf.maximumFractionDigits = 1
    return nf
}()

假设你想打印的变量是

var printVar = 3.567

这将确保它以所需的格式返回:

numberFormatter.StringFromNumber(printVar)

因此，这里的结果是“3.6”(四舍五入)。虽然这不是最经济的解决方案，但我这样做是因为OP提到了打印(在这种情况下String不是不可取的)，并且因为该类允许设置多个参数。

在Swift 5.5和Xcode 13.2中:

let pi: Double = 3.14159265358979
String(format:"%.2f", pi)

例子:

附注:自Swift 2.0和Xcode 7.2以来一直如此

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.

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

Swift最佳方式

这就是我把四舍五入到小数点后两位的方法

let val = round(100 * scale) / 100

Swift 4, Xcode 10

yourLabel.text =  String(format:"%.2f", yourDecimalValue)