I need to format a USA phone number, from the typi...

# random

[Colton Idle]

I need to format a USA phone number, from the typical 10 digits to [phone]

I have this, but it feels a little cumbersome and not kotlin-y, but maybe it's what I have to do. Any suggestions?

fun formatPhoneNumber(original: String): String {
    var number = original
    number =
        number.dropLast(4) +
            "-" +
            number.substring(
                number.length - 4,
                number.length,
            )
    number =
        number.substring(0, number.length - 8) +
            ")" +
            number.substring(
                number.length - 8,
                number.length,
            )
    (number.dropLast(12) +
            "(" +
            number.substring(
                    number.length - 12,
                    number.length,
            )).also { number = it }
    return number
}

[Ruckus]

fun formatPhoneNumber(original: String) = buildString {
    append('(')
    append(original.substring(0, 3))
    append(") ")
    append(original.substring(3, 6))
    append('-')
    append(original.substring(6, 10))
}

[Colton Idle]

<http://com.google.common.io|com.google.common.io>.Files.append ?

[Colton Idle]

Oh. Just saw your edit. A couple of missing parens, but this is what I landed on.

fun formatPhoneNumber(original: String) = buildString {
    append('(')
    append(original.substring(0, 3))
    append(") ")
    append(original.substring(3, 6))
    append('-')
    append(original.substring(6, 10))
}

This does look way better than the original. cheers

[Ruckus]

Yeah, sorry. I was typing from memory. I fixed it 🙂

[Colton Idle]

That's pretty good from memory!

[Matteo Mirk]

Alternative using regex

val numberGroups = """(\d{3})(\d{3})(\d{4})""".toRegex()

fun formatPhoneNumber(original: String) = original.replace(numberGroups, "($1) $2-$3")

[Matteo Mirk]

ps: this message would have been better posted in #C1H43FDRB;)

[thanksforallthefish]

maybe overkilling, but I usually go with https://github.com/google/libphonenumber

[Colton Idle]

Thanks. I will probably go for libphonenumber in the future.

[Matteo Mirk]

yeah, I was about to suggest that lib. Used it in production years ago and does a great job.

[Klitos Kyriacou]

As a long-time Java developer getting into Kotlin, can you tell me why buildString was chosen in the answer above instead of a simple string concatenation operator?

fun formatPhoneNumber(original: String) =
    "(" + original.substring(0, 3) + ") " + original.substring(3, 6) + "-" + original.substring(6, 10)

Is there a performance-related reason or is it just more idiomatic?

[Matteo Mirk]

Performance AND idiomaticism: buildString() gives you a DSL over a StringBuilder instance. Your code allocates 7 temporary strings before creating the final one. Of course its weight is negligible if “original” is short and the function isn’t executed in a hot loop or thousands time/sec. But anyway using that function, or a template string would be more idiomatic Kotlin.

[Klitos Kyriacou]

Thanks, but is it really a performance gain to use buildString? In Java, stringA + stringB + stringC performs just as fast as using StringBuilder because the Java compiler generates the same bytecode, without creating any intermediate temporary string objects. It used to generate intermediate strings back in the days before Java 1.7. If Kotlin generates intermediate strings, doesn't this sound a little like a backward step?

[Matteo Mirk]

Uh yes, my bad… I knew about the JDK 7 optimization, and that still applies if you use Kotlin/JVM. But keep in mind that Kotlin is a multi-platform language, so this couldn’t apply to other targets. As for using buildString() I guess it depends on the situation: the lambda will let you write additional construction logic while calling append() in a short way. As for me I always prefer string templates as the most idiomatic form, so I would have written that as

"(${original.substring(0, 3)}) ${original.substring(3, 6)}-${original.substring(6, 10)}"

But in this case I prefer best my solution 😛 https://kotlinlang.slack.com/archives/C09222272/p1628233961007000?thread_ts=1628180541.005400&cid=C09222272

[thanksforallthefish]

since java 9 the replacement with string builder is replace indified string concatenation (https://openjdk.java.net/jeps/280). ofc this is all jit magic, so I am not exactly sure under which circumstances it happens, but one of the reason I like jvm is that you don’t care until you don’t care. as in, write the simplest code you can, don’t care about performance optimization (beyond the obvious one, don’t sort O(n^2) ) until you don’t need to care.

also why I am still not fully sold on multiplatform, but I really really need to try harder and more seriously