Precision String Format Specifier In Swift

Question

Below is how I would have previously truncated a float to two decimal places

NSLog(@\" %.02f %.02f %.02f\", r, g, b);

I checked the docs an

Answer 1

You can't do it (yet) with string interpolation. Your best bet is still going to be NSString formatting:

println(NSString(format:"%.2f", sqrt(2.0)))

Extrapolating from python, it seems like a reasonable syntax might be:

@infix func % (value:Double, format:String) -> String {
    return NSString(format:format, value)
}

Which then allows you to use them as:

M_PI % "%5.3f"                // "3.142"

You can define similar operators for all of the numeric types, unfortunately I haven't found a way to do it with generics.

Swift 5 Update

As of at least Swift 5, String directly supports the format: initializer, so there's no need to use NSString and the @infix attribute is no longer needed which means the samples above should be written as:

println(String(format:"%.2f", sqrt(2.0)))

func %(value:Double, format:String) -> String {
    return String(format:format, value)
}

Double.pi % "%5.3f"         // "3.142"