[string-formatting] Format in kotlin string templates

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"?

It's simple, use:

val str: String = "%.2f".format(3.14159)

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}")

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

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

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)