How to use println in Swift to format number

问题:

When logging-out a float in Objective-C you can do the following to limit your output to only 2 decimal places:

float avgTemp = 66.844322156
NSLog (@"average temp. = %.2f", avgTemp);

But how do you do this in Swift? And how do you escape other characters in println in Swift?

Here's a regular Swift println statement:

println ("Avg. temp = \(avgTemp)")

So how do you limit decimal places?

Also, how do you escape double-quotes in println?

回答1:

Here's the shortest solution I found thus far:

let avgTemp = 66.844322156
println(NSString(format:"%.2f", avgTemp))

Its like the swift version of NSString's stringWithFormat

回答2:

Everything about the format of a number as a string can be adjusted using a NSNumberFormatter:

let nf = NSNumberFormatter()
nf.numberStyle = NSNumberFormatterStyle.DecimalStyle
nf.maximumFractionDigits = 2
println(nf.stringFromNumber(0.33333)) // prints 0.33

You can escape quotes with a backslash

println("\"God is dead\" -Nietzsche")

回答3:

Println() is deprecated.

 var avgTemp = 66.844322156
  print("average temp. = (round(avgTemp*100)/100)")   // average temp. = 66.84


  //or

 print(NSString(format:"average temp. = %.2f", avgTemp))) // average temp. = 66.84 
 avgTemp = 66.846322156

 print(String(format:"average temp. = %.2f", avgTemp))  // average temp. = 66.85 

回答4:

If you need to print floating point numbers often with a certain precision, you could extend Float and Double with convenience methods. For example, for 2 significant figure precision:

// get Float or Double with 2 significant figure precision
var numberFormatter = NSNumberFormatter()
extension Float {
    var sf2:String {
        get {
            numberFormatter.numberStyle = NSNumberFormatterStyle.DecimalStyle
            numberFormatter.maximumSignificantDigits = 2
            return numberFormatter.stringFromNumber(self)!
        }
    }
}
extension Double {
    var sf2:String {
        get {
            numberFormatter.numberStyle = NSNumberFormatterStyle.DecimalStyle
            numberFormatter.maximumSignificantDigits = 2
            return numberFormatter.stringFromNumber(self)!
        }
    }
}

Then when you need to print things:

let x = 5.23325
print("The value of x is \(x.sf2)")