Kotlin. How to convert string to Int without loosing precision?

Question

I'm trying to convert String to Int. The String can contain number as Int or as Double. But I need to convert string to Int anyway.

Here is my code:

val str = "999.13"
val number = str.toDoubleOrNull()?.roundToInt() ?: 0 // number will be 999

It works but there is one problem. If the source string will contain a very large number, for example 99999999999, then I get an incorrect number. After casting the string to a double, I lose precision.

What is the best way to perform such a manipulation without loss of precision? I would like to refrain from using BigDecimal, BigInteger etc.

Perhaps there is a more elegant solution for kotlin, please help me.

Answer 1

There's no way for Double and Long to hold bigger values than their largest possible values, so of course you will lose precision. That's why BigDecimal/BigInteger exist. They are the only ways to handle numbers that are bigger than the largest values Double and Long can handle, unless you want to handle parsing of the String yourself (note, you are parsing with toDoubleOrNull(), not casting).

I'm not sure why you'd want to avoid BigDecimal, but you could split the number at the decimal place, use toIntOrNull() or toLongOrNull() on the first part of the String and use toFloatOrNull() on the second part so you can round it to either 0 or 1 and add that to the first part to do the rounding:

val result = if ("." !in input)
    input.toIntOrNull()
else {
    val (firstPart, secondPart) = input.split(".")
    val integerPart = firstPart.toIntOrNull()
    integerPart?.let { it + (".$secondPart".toFloatOrNull()?.roundToInt() ?: 0) }
}

It would be a bit easier to use BigDecimal.

val result = runCatching {
    BigDecimal(input).setScale(0, RoundingMode.HALF_UP).toInt()
}.getOrNull()

Both of the above would be simpler if you already know your input is valid.