Switch-When statement

Overview

switch-when statement is a multiple conditional jump. In a condition of when statement, you can put an expression. If the condition’s value in when as a result is same, direct value is checked first. Otherwise, the condition written first is checked first.

You DO NOT need break for exitting when statement because kinx switch-when is automatically break it. Instead, if you need to do fallthrough, put the fallthrough keyword at the last statement in when clause.

switch (x) {
when 1:
    /* ... */
when 2:
    /* ... */
}

fallthrough keyword

You can use fallthrough keyword to avoid an automatic break. The fallthrough keyword can be put anywhere, and it marks not to break the case. Note that the fallthrough keyword never jump anywhere and just mark it.

switch (x) {
when 1:
    /* ... */
    fallthrough;    // marks this case as no break.
    /* ... */       // statements are proceeded and move to the next case after it will be done.
when 2:
    /* ... */
}

Else case

It is not necessary to put otherwise case at the bottom.

switch (x) {
when 1:
    /* ... */
otherwise: /* fallthrough */
    fallthrough;
when 2:
    /* ... */
}

When label

Any value is available for when label.

switch (x) {
when 1:         // Number is available
    /* ... */
when a:         // variable is available
    /* ... */
when x[1]+1:    // expression is available
    /* ... */
when f():       // function call is available, but calling a function will be performed every time when comparing here.
    /* ... */
}

Examples

Example 1. Normal when

Code

function test(a) {
    var r;
    switch (a) {
    when 1: r = a * 2;
    when 2: r = a * 4;
    when 3: r = a * 8;
    when 4: r = a * 16;
    otherwise:
    }
    return r ?? -1;
}
0.upto(8, &(n) => System.println("%d => %3d" % n % test(n)));

Result

0 =>  -1
1 =>   2
2 =>   8
3 =>  24
4 =>  64
5 =>  -1
6 =>  -1
7 =>  -1
8 =>  -1

Example 2. With do-while

Code

function test(count) {
    switch (count) {
    otherwise: do {  System.println("%d" % count); count++; fallthrough;
    when 0:          System.println("%d" % count); count++; fallthrough;
    when 1:          System.println("%d" % count); count++; fallthrough;
    when 2:          System.println("%d" % count); count++; fallthrough;
            } while (count < 8);
    }
}
test(2);

Result

2
3
4
5
6
7
8
9
10

Example 3. Non-integer value

Code

function f() {
    return "a4";
}
function test(a) {
    var r, x = [1, 1.2, "a2", "a"];
    switch (a) {
    when "a1":     r = 2;
    when x[-2]:    r = 4;
    when x[3]+"3": r = 8;
    when f():      r = 16;
    otherwise:
    }
    return r ?? -1;
}
0.upto(8, &(n) => System.println("%d => %3d" % n % test("a" + n)));

Result

0 =>  -1
1 =>   2
2 =>   4
3 =>   8
4 =>  16
5 =>  -1
6 =>  -1
7 =>  -1
8 =>  -1

Example 4. Complex switch-when pattern

Code

var array = [1,2,3,4,5,6,7,8,9,10];
function switchTest(n) {
    switch (n) {
    when 1:     System.println(n);
    when 2:     System.println(n);
    when 3:     System.println(n);
    when 4:     System.println(n);
    when 5:     System.println(n);
    when 6:     System.println(n);
    when 7:     System.println(n);
    when 8:     System.println(n);

    otherwise:
        System.print("default - ");
        fallthrough;
    when 100:
        System.println(n);
    when array.length():
        System.println("array-length:%{n}");
    when "aaa":
        System.println(n);
    when "bbb":
        System.println(n);
    }
}
0.upto(100, function(i) {
    if (12 < i && i <= 97) {
        return; // omitted.
    }
    System.print("%{i} => ");
    switchTest(i);
});

Result

0 => default - 0
1 => 1
2 => 2
3 => 3
4 => 4
5 => 5
6 => 6
7 => 7
8 => 8
9 => default - 9
10 => array-length:10
11 => default - 11
12 => default - 12
98 => default - 98
99 => default - 99
100 => 100