Format in Kotlin string templates

Question

Kotlin has an excellent feature called string templates.

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"?

Is there any multiplatform solution to this? – KenIchi Jan 21 '20 at 07:40 — KenIchi, Jan 21 '20 at 07:40

score 319 · Accepted Answer · edited Apr 15 '22 at 14:01

319

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)

This will work only in Kotlin/JVM.

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

edited Apr 15 '22 at 14:01

cubuspl42

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answered Apr 15 '14 at 15:21

Andrey Breslav

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Is this available now? – Ragunath Jawahar Mar 13 '15 at 08:00
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@RagunathJawahar, the answer is still up-to-date, we didn't improve on it yet – Andrey Breslav Mar 13 '15 at 12:09
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@AndreyBreslav How does String interpolation format float/double values? Is the decimal point locale dependent or always a dot? – Lars Blumberg Jul 17 '15 at 14:58
It calls `Double.toString()` – Andrey Breslav Jul 18 '15 at 21:27
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@AndreyBreslav chance Kotlin could copy C# style of string formatting? Of all of the different formatting types I've used, it's by far the best. – Nick Nov 13 '17 at 22:07
In Kotlin you can do `"%.${digits}f".format(pi)` to be more concise. The literal string is evaluated to a String class. – Simon Featherstone Nov 29 '17 at 16:36
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@AndreyBreslav How about now? Is this available? – cihan adil seven Jun 18 '18 at 14:25
Still would be great to have it. :D – Tobias Reich Nov 22 '18 at 09:31
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Am I mistaken or is this still not available almost 4 years later? – RDM Jan 19 '19 at 08:31
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Add it as a New Year's present for 2020! – mark mark Dec 28 '19 at 19:49
@markmark They have a year and one day, they can do it! – David Callanan Dec 30 '19 at 17:27
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@DavidCallanan nope, still not around. – mcy Mar 05 '20 at 11:40
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@AndreyBreslav If it happens, I think it would be nice to follow Python's syntax `"average: ${average:.4f} (Arithmetic mean)"` – Joschua Aug 22 '20 at 15:16
@HosseinKurd of course it does, if you need the exponent, just use the plain `"$number"` or `number.toString()` – Andrey Breslav Nov 08 '20 at 14:55
This is still not available in 2021? – sangeetds Jun 26 '21 at 04:33
Consider to use `DecimalFormat("0.##").format(this)` instead. It removes trailing zero. – Zufar Muhamadeev Aug 04 '21 at 15:48

score 165 · Answer 2 · edited May 14 '19 at 11:27

165

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)

edited May 14 '19 at 11:27

William Desportes

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answered Nov 03 '15 at 06:22

akhy

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I'm guessing he meant this documentation: https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#format-java.util.Locale-java.lang.String-java.lang.Object...- – stuckj Dec 27 '17 at 05:35
@Rob See also [discussion](https://stackoverflow.com/questions/23086291/format-in-kotlin-string-templates#comment79967924_33492636) on the documentation – Matt Mc Mar 09 '18 at 04:44
Works in Kotlin 1.3.21 – F. P. Freely Apr 09 '19 at 16:58

score 39 · Answer 3 · edited Dec 08 '17 at 09:51

39

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

edited Dec 08 '17 at 09:51

Willi Mentzel

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answered Dec 08 '17 at 00:33

user1767754

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String.Companion.format is not found now in Kotlin v1.2.21. What is the alternative?. – Sai Feb 03 '18 at 09:07

score 32 · Answer 4 · edited Dec 13 '20 at 09:43

32

It's simple, use:

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

edited Dec 13 '20 at 09:43

Willi Mentzel

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answered Aug 25 '18 at 11:53

masoomyf

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Willi Mentzel · Answer 5 · 2017-12-08T09:57:11.470

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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

edited Dec 08 '17 at 09:57

answered Dec 08 '17 at 09:14

Willi Mentzel

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score 3 · Answer 6 · answered Jan 15 '20 at 03:38

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}")

Deniz Babat · Answer 7 · 2022-04-22T10:50:54.157

0

It has string formatting Example in Kotlin for Android TextView:

val format = String.format("<font color=#3177a3> test1: <b>%s</b><br> test2: <b>%s</b><br> test3: <b>%s</b></font>", "value1", "value2", "value3")
textView.text = format

edited Apr 22 '22 at 10:50

answered Apr 22 '22 at 09:25

Deniz Babat

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score -5 · Answer 8 · answered Mar 17 '21 at 20:11

-5

It has string formatting Example:

fun printSum(a: Int, b: Int): Unit {
    println("sum of $a and $b is ${a + b}")
}

answered Mar 17 '21 at 20:11

Sibyl

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Format in Kotlin string templates

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