Kotlin has an excellent feature called string templates. I really love it.
val i = 10
val s = "i = $i" // evaluates to "i = 10"
But is it possible to have any formatting in the templates? For example, I would like to format Double in string templates in kotlin, at least to set a number of digits after a decimal separator:
val pi = 3.14159265358979323
val s = "pi = $pi??" // How to make it "pi = 3.14"?
Unfortunately, there's no built-in support for formatting in string templates yet, as a workaround, you can use something like:
"pi = ${pi.format(2)}"
the .format(n) function you'd need to define yourself as
fun Double.format(digits: Int) = "%.${digits}f".format(this)
There's clearly a piece of functionality here that is missing from Kotlin at the moment, we'll fix it.
As a workaround, There is a Kotlin stdlib function that can be used in a nice way and fully compatible with Java's String format (it's only a wrapper around Java's String.format())
See Kotlin's documentation
Your code would be:
val pi = 3.14159265358979323
val s = "pi = %.2f".format(pi)
Kotlin's String class has a format function now, which internally uses Java's String.format method:
/**
* Uses this string as a format string and returns a string obtained by substituting the specified arguments,
* using the default locale.
*/
@kotlin.internal.InlineOnly
public inline fun String.Companion.format(format: String, vararg args: Any?): String = java.lang.String.format(format, *args)
Usage
val pi = 3.14159265358979323
val formatted = String.format("%.2f", pi) ;
println(formatted)
>>3.14
Its simple, Use:
val str:String = "%.2f".format(3.14159)
Since String.format is only an extension function (see here) which internally calls java.lang.String.format you could write your own extension function using Java's DecimalFormat if you need more flexibility:
fun Double.format(fracDigits: Int): String {
val df = DecimalFormat()
df.setMaximumFractionDigits(fracDigits)
return df.format(this)
}
println(3.14159.format(2)) // 3.14
A couple of examples:
infix fun Double.f(fmt: String) = "%$fmt".format(this)
infix fun Double.f(fmt: Float) = "%${if (fmt < 1) fmt + 1 else fmt}f".format(this)
val pi = 3.14159265358979323
println("""pi = ${pi f ".2f"}""")
println("pi = ${pi f .2f}")
