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

User · Answer

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   gt  gt 3 14

User · Answer

A couple of examples   infix fun Double f fmt  String       fmt  format this  infix fun Double f fmt  Float        if  fmt  lt  1  fmt   1 else fmt f  format this   val pi   3 14159265358979323  println    pi     pi f   2f       println  pi     pi f  2f

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

It s simple  use  val str  String    quot   2f quot  format 3 14159

[string-formatting] Format in kotlin string templates

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