Variables, types and strings

A game on your phone remembers your name, your points, your level and whether the sound is on. These are different kinds of data: text, a whole number, a yes/no answer. A program keeps such data in variables, and Kotlin carefully tracks the type of every variable so that you never divide a piece of text by a number by mistake.

val and var

In Kotlin a variable is created with one of two words. val (value) is a read-only variable: it gets a value once and cannot be changed afterwards. var (variable) can receive a new value at any time. The = sign assigns a value.

fun main() {
    val name = "Aysel"
    var score = 10
    println(name)
    println(score)
    score = score + 5
    score += 3
    println(score)
}

Aysel
10
18

Basic types and type inference

You do not have to write the type every time: when you write val age = 15, the compiler works out by itself that age is an Int. This is called type inference. If you like, you can also state the type explicitly after a colon: val age: Int = 15. To make big numbers easier to read, you can separate digits with underscores: 1_000_000.

fun main() {
    val age: Int = 15
    val height = 1.62
    val isStudent = true
    val grade = 'A'
    val worldPopulation = 8_000_000_000L
    println(age)
    println(height)
    println(isStudent)
    println(grade)
    println(worldPopulation)
}

15
1.62
true
A
8000000000

Arithmetic and type conversion

Kotlin has the operators +, -, *, / and %, which gives the remainder. Watch out: dividing two Int values also gives an Int, and the fractional part is dropped — 7 / 2 gives 3. If you need a fractional result, one of the numbers must be a Double. Kotlin never converts types on its own; you do it explicitly with functions such as toInt(), toDouble() and toString().

fun main() {
    val apples = 7
    val kids = 2
    println(apples / kids)
    println(apples % kids)
    println(apples / 2.0)
    println(apples.toDouble() / kids)
    val price = "25".toInt()
    println(price * 2)
    println(9.99.toInt())
}

3
1
3.5
3.5
50
9

Strings and templates

You can glue text together with +, but Kotlin has a more convenient way — string templates. Inside a string you write $ followed by a variable name, and its value is inserted automatically. For a whole expression, use curly braces: ${age + 1}.

fun main() {
    val name = "Murad"
    val city = "Baku"
    val age = 14
    println("$name lives in $city.")
    println("Next year $name will be ${age + 1}.")
    println("The name has ${name.length} letters.")
    println("Hello, " + name + "!")
}

Murad lives in Baku.
Next year Murad will be 15.
The name has 5 letters.
Hello, Murad!

Strings come with many useful functions. length gives the length, [0] takes the first character (counting starts at zero), and uppercase() and lowercase() change the letter case:

fun main() {
    val word = "Kotlin"
    println(word.length)
    println(word.uppercase())
    println(word.lowercase())
    println(word[0])
    println(word.last())
    println(word.contains("lin"))
    println(word.reversed())
    println("ha".repeat(3))
    println("  hello  ".trim() + "!")
}

6
KOTLIN
kotlin
K
n
true
niltoK
hahaha
hello!