Swift number formatting

Question

I am just starting to get to know Swift but I am having a serious problem with number formatting at an extremely basic level.

For example, I need to display an integer with at least 2 digits (e.g. 00, 01, 02, 03, 04, 05 ...). The normal syntax I'd expect would be something like:

println(" %02i %02i %02i", var1, var2, var3);

...but I don't find any clear instruction for how to achieve this in Swift. I find it really hard to believe that I need to create a custom function to do that. The same for returning a float or double value to a fixed number of decimal places.

I've found links to a couple of similar questions (Precision String Format Specifier In Swift & How to use println in Swift to format number) but they seem to mix objective C and even talk about Python and using unity libraries. Is there no Swift solution to this basic programming need? Is it really true that something so fundamental has been completely overlooked in Swift?

Antonio · Accepted Answer · 2014-10-02T19:03:29.653

17

You can construct a string with a c-like formatting using this constructor:

String(format: String, arguments:[CVarArgType])

Sample usage:

var x = 10

println(String(format: "%04d", arguments: [x])) // This will print "0010"

If you're going to use it a lot, and want a more compact form, you can implement an extension like this:

extension String {
    func format(arguments: [CVarArgType]) -> String {
        return String(format: self, arguments: arguments)
    }
}

allowing to simplify its usage as in this example:

"%d apples cost $%03.2f".format([4, 4 * 0.33])

edited Oct 02 '14 at 19:03

answered Oct 02 '14 at 18:54

Antonio

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1

Thanks @MartinR , that's much better :) - added an even shorter form using an extension. – Antonio Oct 02 '14 at 19:12
1

Thanks a lot to both of you. After reading pages and pages of discussion around this, its great to have a clear, concise solution. – user3535074 Oct 02 '14 at 19:38
@user3535074 don't forget to import Foundation to make it works as expected – user3441734 Dec 17 '15 at 17:39

score 7 · Answer 2 · answered Sep 21 '16 at 13:34

Here's a POP solution to the problem:

protocol Formattable {
    func format(pattern: String) -> String
}
extension Formattable where Self: CVarArg {
    func format(pattern: String) -> String {
        return String(format: pattern, arguments: [self])
    }
}
extension Int: Formattable { }
extension Double: Formattable { }
extension Float: Formattable { }

let myInt = 10
let myDouble: Double = 0.01
let myFloat: Float = 1.11

print(myInt.format(pattern: "%04d"))      // "0010
print(myDouble.format(pattern: "%.2f"))   // "0.01"
print(myFloat.format(pattern: "$%03.2f")) // "$1.11"
print(100.format(pattern: "%05d"))        // "00100"

score 0 · Answer 3 · answered Jun 02 '16 at 02:32

0

You can still use good ole NSLog("%.2f",myFloatOrDouble) too :D

answered Jun 02 '16 at 02:32

CommaToast

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score 0 · Answer 4 · answered Oct 28 '16 at 17:41

There is a simple solution I learned with "We <3 Swift", which is so easy you can even use without Foundation, round() or Strings, keeping the numeric value.

Example:

var number = 31.726354765
var intNumber = Int(number * 1000.0)
var roundedNumber = Double(intNumber) / 1000.0

result: 31.726

Swift number formatting

4 Answers4

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