joffrey-bion / kotlin-lessons

A set of small programs with increasingly difficult goals to teach Kotlin to absolute programming beginners

Kotlin 101

Basic syntax

The program starts at the main() function:

fun main() {
    // The execution starts here, each line will be executed sequentially
    println("Hello, world!")
}

The println function prints a line of text to the screen. Using print instead doesn't go to the next line, so we can keep printing things on the same line:

print("Hello")
println("World!")
// prints "Hello, World!" to the screen on the same line, and then goes to the next line

Any text after // is considered a comment and ignored by the compiler (until the end of the line).

// this way we can explain things!

/*
For long text on multiple lines,
we can do this instead.
 */

Declare a variable with the var keyword, and assign values with =:

var x = 42  // we declare the variable with the `var` keyword
x = 15      // we can then assign new values to it later
x = 12 + 5  // values can be expressions
x = x + 1   // we can even reuse the current value of the variable itself
x += 5      // equivalent to x = x + 5
x++         // equivalent to x = x + 1

Declare a read-only variable (a simple value) with the val keyword:

val x = 42

x = 13 // ERROR: this is forbidden, variables declared with 'val' are read-only

Variables can hold values of other types than integers:

val decimal = 42.0
val character = 'C'      // note the single quotes around it
val string = "some text" // note the double quotes around it

The types of these variables were inferred so far, but we can specify them explicitly:

val integer: Int = 42
val decimal: Double = 42.0 // "Double" stands for "Double-precision floating point"
val character: Char = 'C'
val string: String = "some text"

This is how to declare a function named add() with the parameters a and b which are both integers (type Int). It also returns an integer value, hence the : Int at the end of the first line. The braces delimit the body of the function

//      parameters     return type
//       /      \         |  
fun add(a: Int, b: Int): Int {
    val result = a + b
    return result // exit point of the function
}

// now we can call the function this way
val r1 = add(42, 12)
// we can also name the arguments for clarity
val r2 = add(a = 42, b = 12)

Types

val integer = 42
val char = 'C'
val string = "some text" // or rather: string of characters. This is just text