Rounding a double value to x number of decimal places in swift

Question

Can anyone tell me how to round a double value to x number of decimal places in Swift?

I have:

var totalWorkTimeInHours = (totalWorkTime/60/60)

With totalWorkTime being an NSTimeInterval (double) in second.

totalWorkTimeInHours will give me the hours, but it gives me the amount of time in such a long precise number e.g. 1.543240952039......

How do I round this down to, say, 1.543 when I print totalWorkTimeInHours?

Answer 1

You can use Swift's round function to accomplish this.

To round a Double with 3 digits precision, first multiply it by 1000, round it and divide the rounded result by 1000:

let x = 1.23556789
let y = Double(round(1000 * x) / 1000)
print(y) /// 1.236

Unlike any kind of printf(...) or String(format: ...) solutions, the result of this operation is still of type Double.

EDIT:
Regarding the comments that it sometimes does not work, please read this: What Every Programmer Should Know About Floating-Point Arithmetic

Answer 2

Extension for Swift 2

A more general solution is the following extension, which works with Swift 2 & 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
    }
}

Extension for Swift 3

In Swift 3 round is replaced by rounded:

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
    }
}

Example which returns Double rounded to 4 decimal places:

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

Answer 3

How do I round this down to, say, 1.543 when I print totalWorkTimeInHours?

To round totalWorkTimeInHours to 3 digits for printing, use the String constructor which takes a format string:

print(String(format: "%.3f", totalWorkTimeInHours))