Kotlin – Format in kotlin string templates

kotlinstring-formatting

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"?

Best 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.

Related Solutions

C# – Float to String format specifier

Use ToString() with this format:

12345.678901.ToString("0.0000"); // outputs 12345.6789
12345.0.ToString("0.0000"); // outputs 12345.0000

Put as much zero as necessary at the end of the format.

Python – String formatting: % vs. .format vs. f-string literal

To answer your first question... .format just seems more sophisticated in many ways. An annoying thing about % is also how it can either take a variable or a tuple. You'd think the following would always work:

"hi there %s" % name

yet, if name happens to be (1, 2, 3), it will throw a TypeError. To guarantee that it always prints, you'd need to do

"hi there %s" % (name,)   # supply the single argument as a single-item tuple

which is just ugly. .format doesn't have those issues. Also in the second example you gave, the .format example is much cleaner looking.

Why would you not use it?

not knowing about it (me before reading this)
having to be compatible with Python 2.5

To answer your second question, string formatting happens at the same time as any other operation - when the string formatting expression is evaluated. And Python, not being a lazy language, evaluates expressions before calling functions, so in your log.debug example, the expression "some debug info: %s"%some_infowill first evaluate to, e.g. "some debug info: roflcopters are active", then that string will be passed to log.debug().

Best Answer

Related Solutions

C# – Float to String format specifier

Python – String formatting: % vs. .format vs. f-string literal

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