Like if statements, A switch statement controls program flow by allowing you to specify different code that should be executed under various cases.
The general syntax looks like this:
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switch (var) {
case val1:
// statements
break;
case val2:
// statements
break;
...
default:
// statements
}
Where var is a variable whose value to investigate, and the val1, val2 after each case are constant values that var might be.
A switch statement compares the value of a variable to the values specified in case statements. When a case statement is found whose value matches that of the variable, the code in that case statement is run.
Here’s a more concrete example:
switch (var) {
case 1:
doThing1();
break;
case 2:
doThing2();
break;
}
afterTheSwitch();
In the above example, if var == 1, then the code beginning on the line after case 1 gets executed. That is, if var is one, doThing1() gets called first, and then the break statement is executed.
The break keyword exits the switch statement, and is typically used at the end of each case. Since there is a break at the end of case 1, the switch statement exits, and the next line to be run is the one which calls afterTheSwitch().
Without a break, the switch statement will continue executing the following case expressions (“falling-through”) until a break (or the end of the switch statement) is reached. Let’s pretend the switch looked like this instead:
switch (var) {
case 1:
doThing1();
// no break statement anymore
case 2:
doThing2();
break;
}
afterTheSwitch();
Now, if var is one, doThing1() gets executed like before. However, without a break, the code would continue to be executed line-by-line, so doThing2() would be called next. At this point, a break has been reached, so the program continues by calling afterTheSwitch(). This is usually not what you want, which is why each case usually has a break at the end.
Writing “default:” instead of a case statement allows you to specify what to do if none of the case statements matches. Having a default is optional (you can leave it out), but if you have one, it must appear after all of the case statements. Let’s add a default to the switch we’ve been discussing:
switch (var) {
case 1:
doThing1();
break;
case 2:
doThing2();
break;
default:
doSomethingElse();
}
afterTheSwitch();
If var is one, then doThing1() gets called. If var is two, doThing2() gets called. If var is anything else, doSomethingElse() gets called. As stated above, a default is optional. If you’re missing one and none of the case statements match, the switch does nothing at all, as if it weren’t there.
switch statements are often used with an enum value as the variable to compare. In this case, you can write down all of the values the enum takes as case statements, and be sure you’ve covered all the possibilities.
License and Attribution
Portions of this page were adapted from the Arduino Reference Documentation, which is released under a Creative Commons Attribution-ShareAlike 3.0 License.