Kotlin Variables and Data Types

A variable is a named box that stores a value, such as a name, a price or a mark.

val and var

Kotlin has two keywords for creating variables:

fun main() {
    val name = "Ayesha"   // cannot be changed
    var age = 19          // can be changed
    age = 20
    println("$name is $age years old")
}

Ayesha is 20 years old

If you try name = "Sara", Kotlin shows an error, because name was made with val.

Type Inference

In the example above we never wrote a type. Kotlin looked at the value and inferred it: "Ayesha" is a String and 19 is an Int. You can also write the type yourself after a colon:

fun main() {
    val price: Int = 250
    val quantity: Int = 4
    val total = price * quantity
    println("Total: Rs $total")
}

Total: Rs 1000

Data Types

A Char uses single quotes. A String uses double quotes.

Type Conversion

Kotlin does not change one type into another by itself. You convert with functions such as toInt(), toDouble() and toString():

fun main() {
    val text = "42"
    val number = text.toInt()
    println(number + 8)

    val marks = 75
    val percent = marks.toDouble() / 100
    println(percent)

    println(marks.toString() + " marks")
}

50
0.75
75 marks

Null Safety

null means "no value". In many languages, using a null value by mistake crashes the program. Kotlin prevents this: a normal variable can never hold null.

To allow null, add a question mark after the type, like String?.

fun main() {
    var city: String? = null
    println(city?.length)
    println(city ?: "Unknown")

    city = "Karachi"
    println(city?.length)
}

null
Unknown
7

• ?. is the safe call. If the value is null, it returns null instead of crashing.
• ?: is the Elvis operator. It gives a default value when the left side is null.
• !! tells Kotlin "I am sure this is not null". If you are wrong, the program crashes, so use it rarely.

Constants

For values known before the program runs, use const val at the top of the file:

const val INSTITUTE = "Workownix"

fun main() {
    println("Welcome to $INSTITUTE")
}

Welcome to Workownix

Exercise