Kotlin: Variables

Declaring variables

You declare a variable in Kotlin with the val keyword, the name of the variable, the datatype, and then the actual value the variable will hold.

// val name: dataType = actual value;
val count: Int = 2

To reference a variable in a string, you use the $ symbol.

fun main(){
    val count: Int = 2
    println("You have $count unread messages")
}

Type inference

Instead of specifying the datatype of the variable, we can just declare the variable directly and the compiler will infer the type for you.

val count = 2 // the compiler will infer this as an int

If you need to update the value of a variable, declare it with the Kotlin keyword var, instead of val.

• val — use when you expect the variable value will not change (fixed value).
• var — use when you expect the variable value can change (mutable value).

In Kotlin, it’s preferred to use val over var because it’s safer and more predictable.

fun main(){
    var cartTotal = 0
    cartTotal = 20
    println("Your cart total is $cartTotal")
}

Arrays

To create arrays in Kotlin, you can use functions such as arrayOf(), arrayOfNulls(), or emptyArray(), or the Array constructor.

val numbers = arrayOf(1, 2, 3, 4, 5)
println(numbers.joinToString()) // 1, 2, 3, 4, 5
println(numbers[0]) // 1

Similarly, you can use arrayOfNulls():

val nullArray: Array<Int?> = arrayOfNulls(5)
println(nullArray.joinToString()) // null, null, null, null, null

Empty arrays can be created with:

var exampleArray = emptyArray<Int>()

The Array constructor takes the array size and a function that returns values for array elements given its index:

val initArray = Array<Int>(3) { 0 }
println(initArray.joinToString()) // 0, 0, 0

// creates an Array<String> with values ["0", "1", "2"]
val asc = Array(3) { i -> (i * i ).toString()}
asc.forEach { println(it) }

To check whether two arrays have the same elements, you can use the contentEquals() and contentDeepEquals() functions:

val simpleArray = arrayOf(1, 2, 3)
val simpleArray2 = arrayOf(1, 2, 3)

println(simpleArray.contentEquals(simpleArray2)) // true