Type check and conversion

I’m a beginner in Kotlin programming and I’m trying to understand how the “if” function works.
Following is a code I’m trying to fix, some suggestion and explanation from you?

    fun sumNumbers(num1: Double, num2: Double): Double{
    if(num1 || num2 is Int){
    return num1 + num2

fun main(args:Array<String>) {
    var somma = sumNumbers(5, 34)

if is not a function, it is a keyword that executes its block only if the condition inside of it is true.

This will never work cuz 1) you can’t have 2 conditions in one using ||, so it’ll need to be if(num1 is Int || num2 is Int) 2) This is an ||, but it seems as though you actually want a && because you are converting both numbers and finally 3) a Double can never be cast as an Int, because they are totally different types. To be honest, I don’t really see the purpose of that function. Did you maybe just want to add 2 Ints? if so, then make num1 and num2 Ints. Could you please clarify what you are trying to achieve?

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Here’s an example for a function summing two numbers:

fun sumNumbers(num1: Double, num2: Double): Double = num1 + num2

fun main() {
    val sum = sumNumbers(5.0, 34.0)

Just as a reminder, when we say Integer and Double, we aren’t talking about numbers like we do learning math–instead we’re talking about types. It’s reasonable to say whole numbers and integers are a subset of rational numbers. An Int in programming is not a subtype of Double.

Checking if an Double is an Int is not checking if it’s a whole number. You might as well check if a Double is a File or String

import java.io.File
fun main() {
5.0 is File
5.0 is String
5.0 is Int

If what you really want is to check if a Double is a whole number, you can do that a few different ways. Here’s an example using compareTo:

fun isWholeNumber(num: Double): Boolean {
    return num.compareTo(num.toInt()) == 0 

fun main() {
    val wholeDouble = 5.0

    val nonWholeDouble = 5.0000001

so, how can I code a function that in case of the value is double returns double and in case of the value is integer returns an integer?

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You have a few options but you probably want to define two functions with the same name (aka function overloading)

fun sumNumbers(num1: Int, num2: Int): Int = num1 + num2
fun sumNumbers(num1: Double, num2: Double): Double = num1 + num2

You can use the Number supertype if you want to support anything that can be converted to the basic number types.

fun sumNumbers(num1: Number, num2: Number): Double {
    return num1.toDouble() + num2.toDouble()

Docs for basic number types: https://kotlinlang.org/docs/reference/basic-types.html

You can also use generic function to do this.

// The return type must be the same as param type. Anything can be used for the T type.
fun <T> exampleGenericFunction(param: T): T {
   return TODO("Return some type of T")

Here’s the reference page on generics: https://kotlinlang.org/docs/reference/generics.html

EDIT: Removed some extra stuff as it’s probably misleading for this kind of simple stuff.


Thanks so much, very clear and helpful answer!!!

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