Switch case

One value, some cases, a default — and what it works on.

A chain of if/else if quickly becomes unreadable when the same value is tested against ten possibilities. switch replaces it.

@entry(start)
object MyProgram {
    start() {
        var day := 2;

        switch (day) {
            case 1:
                Console.print(text: "Monday");
                break;
            case 2:
                Console.print(text: "Tuesday");
                break;
            default:
                Console.print(text: "some other day");
        }
    }
}

One value, some cases, a way out

switch (day) evaluates the value once, then looks for the case that matches it. Found, it runs what follows. None matches, it takes the default.

The break closes the case: it makes sure the cases below it are not run.

The default is not compulsory, but leaving it out has a consequence: a value no case foresees then does nothing at all, silently. Writing it, if only to flag the unexpected, costs two lines.

What it works on

Integers and texts, nothing else. A switch on a text compares contents, like the = of section 3:

switch (name) {
    case "simon":
        Console.print(text: "known");
        break;
    default:
        Console.print(text: "unknown");
}

There is no switch on an object. Branching on one is written with if.

Your turn

• Give day a value no case foresees: the default answers.
• Remove the break from case 1: and set day to 1: two lines appear.
• Remove the default and give an unforeseen value: the program prints nothing, and does not complain. That is exactly the silence a default avoids.
• Rewrite the same choice as if/else if and compare the two as reading.