val and var

Kotlin gives you two ways to introduce a name, and it very much wants you to pick the boring one. By the end of this lesson you can choose between them, read a number from input, and update a value as the program runs.

fun main() {
    val name = "Ada"
    var visits = 0

    visits = visits + 1
    visits += 1

    println(name + " has visited " + visits + " times")
}

//> Ada has visited 2 times

visits + 1 is ordinary addition, and visits += 1 is shorthand for visits = visits + 1. The Numbers lesson covers arithmetic properly; here the point is the keyword. Joining text and a number with + works as long as the text comes first.

Try to reassign a val and the program does not build:

val name = "Ada"
name = "Grace"   // error: val cannot be reassigned

That is a compile error, not a runtime one. The bug never reaches anybody.

What does val actually freeze?

val freezes the name, not the thing the name points at. A val is a label glued to one object. The label cannot be moved to a different object; if the object itself has moving parts, those parts still move. A String and an Int have no moving parts, so the distinction never bites here. It matters for collections, which the Lists and Arrays lesson picks up.

Types: inferred, but still there

Kotlin is statically typed. Every name has a type fixed at compile time; you just rarely have to write it down.

val city = "Oslo"      // String
val year = 2016        // Int
val ratio = 0.75       // Double
val ready = true       // Boolean
val initial = 'K'      // Char — single quotes, exactly one character

You must write the type when you declare a name before you have a value for it, because there is nothing to infer from:

val label: String
label = "ready"

The core types are Int, Long, Double, Float, Boolean, Char and String. Kotlin has no int versus Integer split like Java's: Int is one type, and the compiler uses a machine primitive underneath wherever it can.

✓ / ✗

✓ Reach for val first and let the compiler tell you when you actually need var:

val label = "total"

✗ Declaring everything var "just in case" throws away help you were being given for free:

var label = "total"    // nothing ever reassigns it

Reading input, and that !!

Every exercise reads standard input with readLine(). It hands back a String? — a string or nothing at all, because the input may have run out. The !! in the starters means "I promise a line is there; blow up if I am wrong":

val count = readLine()!!.toInt()

readLine()!! gives you a plain String, and .toInt() turns the text "7" into the number 7. Skip the conversion and you still hold text, and + on text glues rather than adds. The Null Safety lesson takes !! apart and shows the safer alternatives; for now read it as the starter's way of saying "the input is guaranteed".

Common mistakes

• Reassigning a val. The compiler refuses; change the declaration to var only if the value really must change.
• Dropping .toInt(). The name then holds text, and + glues the pieces together instead of adding them: "7" + "5" is 75.
• Updating the wrong direction. current -= amount and current += amount look alike and give opposite answers; read the sign out loud.

Your exercise

Stock Count reads three whole numbers and keeps a running figure in a var. The starter reads all three and converts them for you; you update the running figure twice and print it. The hidden tests send a negative result and a zero result, so print whatever number comes out rather than assuming it is positive.