Format in kotlin string templates

Question

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"?

Answer 1

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) = java.lang.String.format("%.${digits}f", this)

There's clearly a piece of functionality here that is missing from Kotlin at the moment, we'll fix it.

Answer 2

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)

Answer 3

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

Answer 4

Its simple, Use:

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

Answer 5

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

Answer 6

Try take. It returns a list containing first n elements.

val pi = 3.14159265358979323
val s = pi.take(4)

But it's just about the collections as rekire said, who politely corrected me in the comment below.

You can convert to string, use take and convert back to Double.

val a = 3.14159265358979323
val b = a.toString().take(4).toDouble()

Not the best idea though but it works.