How to use Kotlin when to find if a string is numeric?

Question

I'd like to use a when() expression in Kotlin to return different values from a function. The input is a String, but it might be parsable to an Int, so I'd like to return the parsed Int if possible, or a String if it is not. Since the input is a String, I cannot use the is type check expression.

Is there any idiomatic way to achieve that?

My problem is what the when() expression should look like, not about the return type.

Answer 1

Version 1 (using toIntOrNull and when when as requested)

fun String.intOrString(): Any {
    val v = toIntOrNull()
    return when(v) {
        null -> this
        else -> v
    }
}

"4".intOrString() // 4
"x".intOrString() // x

Version 2 (using toIntOrNull and the elvis operator ?:)

when is actually not the optimal way to handle this, I only used when because you explicitely asked for it. This would be more appropriate:

fun String.intOrString() = toIntOrNull() ?: this

Version 3 (using exception handling):

fun String.intOrString() = try { // returns Any
   toInt()
} catch(e: NumberFormatException) {
   this
}

Answer 2

The toIntOrNull function in the kotlin.text package (in kotlin-stdlib) is probably what you're looking for:

toIntOrNull

fun String.toIntOrNull(): Int? (source)

Platform and version requirements: Kotlin 1.1

Parses the string as an Int number and returns the result or null if the string is not a valid representation of a number.

fun String.toIntOrNull(radix: Int): Int? (source)

Platform and version requirements: Kotlin 1.1

Parses the string as an Int number and returns the result or null if the string is not a valid representation of a number.

More information: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.text/to-int-or-null.html

Answer 3

Using let for one

fun isInteger(str: String?) = str?.toIntOrNull()?.let { true } ?: false

Answer 4

Simple and intuitive

fun isNumeric(str: String) = str.all { it in '0'..'9' }

As @coolMind point out, if you want to filter +/-

fun isNumeric(str: String): Boolean = str
        .removePrefix("-")
        .removePrefix("+")
        .all { it in '0'..'9' }

The performance would be similar

Answer 5

If you want to check if it is numeric (Int) the string and do something a simple solution could be:

if (myString.toIntOrNull() != null) {
   //Write your code you want to execute if myString is (Int)
} else {
   //Write your code you want to execute if myString is (not Int)
}