The `default` case of a `switch` statement doesn't have to be the last one

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Non-last default cases are confusing, but sometimes it makes sense to place them at the beginning or in the middle. Let's see how.

Table of contents

Huh, that's possible?

If you think about all of the switch statements you have ever seen in your life, I guess that in most of them, the default case has been the last or almost last one.

I still remember how dumbfounded I was when a friend of mine sent me the following snippet of Unity code (C#) that he had copy-pasted picked up from a course he was following at the time:

void Update()
{
switch (state)
{
default:
case State.WaitingToStart:
if (Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0))
{
state = State.Playing;
rb.bodyType = RigidBodyType2D.Dynamic;
Jump();
}
break;
case State.Playing:
if (Input.GetKeyDown(KeyCode.Space) || Input.GetMouseButtonDown(0))
{
Jump();
}
break;
case State.Dead:
break;
}
}

As you can see, the default case is at the very top.

My initial thought was: dude, wtf, does this even compile?

I was so used to seeing the default case at the bottom that I couldn't comprehend this. It looked like an if statement where the else branch is at the beginning.

I think my inner logic circuit deduced that since the default case is the first one, the other cases are not even checked.

But turns out that's not the case (no pun intended). The code above is perfectly valid and even makes sense when you think about it. (We'll come back to this code snippet in the What about the Unity code? section.)

The position of the default case doesn't matter (unless there's a fallthrough)

After recovering from the shock of seeing that Unity code, I had to do some digging.

Like the old saying goes, "all programming-related Google searches lead to Stack Overflow." Here's the highest-voted relevant SO question that I could find. Let's see what the accepted answer by Secure says:

The C99 standard is not explicit about this, but taking all facts together, it is perfectly valid.

A case and default label are equivalent to a goto label. [...]

All cases are evaluated, then it jumps to the default label, if given.

(The actual answer is more thorough with excerpts from the C99 standard.)

Here's what's said about JavaScript's switch statements on MDN:

A switch statement first evaluates its expression. It then looks for the first case clause whose expression evaluates to the same value as the result of the input expression [...]

If no matching case clause is found, the program looks for the optional default clause [...] By convention, the default clause is the last clause, but it does not need to be so.

One more: here's what Paul Dixon has answered to a relevant SO question in the context of PHP and o'boy do I agree:

It is an unusual idiom, it causes a little pause when you're reading it, a moment of "huh?". It works, but most people would probably expect to find the default case at the end.

Okay, you get the point: the position of the default case doesn't matter in C, C#, JavaScript, PHP, and probably many other C-style languages as well – if there are no fallthroughs.

If there's a fallthrough (one or more) in the switch statement (no break or return statement between cases), the position of the default case might matter. Fallthroughs are a common concept in switch statements, but if it's still confusing how they work with non-last default cases, the following code examples should clarify this.

Situations where a non-last default case can be useful

While putting the default case to the end is usually the most intuitive option, I found four situations where a non-last default case can be useful.

(If you can think of other useful situations, please tell me!)

Cases in logical order

Let's say you have cases from 1 to 3, and the default case is number 2.

If you specify the cases so that the default case is the last one, the order would be 1, 3 and 2/default. It might instead be more intuitive to specify the cases in logical order: 1, 2/default and 3. Like so:

switch (foo) {
case 1:
// do something...
break;
case 2:
default:
// do something...
break;
case 3:
// do something...
break;
}

The more cases there are and the longer they are, the more sense this probably makes. On the other hand, if the default case is in the middle of a long and complex switch statement, it might be easy to miss it.

I got this idea from an SO answer by Salil.

Normal case first

Let's say you have a situation where there's a normal/default case and a bunch of error cases.

If you put the normal case to the end, it emphasizes the error cases. Unnecessarily so, especially if the error cases are uncommon.

I got this idea from an SO answer by kriss which included this code example:

switch (poll(fds, 1, 1000000)) {
default:
// here goes the normal case : some events occured
break;
case 0:
// here goes the timeout case
break;
case -1:
// some error occurred, you have to check errno
}}

kriss also talks about how the order of cases can affect performance, but I'll just skip over this since it smells like premature optimization. (Maybe it can be important if you are working with low-level, performance-critical code and not thinking about this prematurely.)

Flow control with fallthroughs

If you are using a switch statement for flow control (a series of steps) and don't break between cases, putting default to the beginning makes for a clear switch statement.

The following code example is adapted from an SO answer by DanielM:

switch (step) {
default:
case STEP_1:
// do some stuff for step one
// fallthrough
case STEP_2:
// this follows on from step 1 or you can skip straight to it
}

Another good example is "a state machine where an invalid state should reset the machine and proceed as though it were the initial state," from an SO answer by supercat:

switch (widget_state) {
default: /* Fell off the rails--reset and continue */
widget_state = WIDGET_START;
/* Fall through */
case WIDGET_START:
// ...
break;
case WIDGET_WHATEVER:
// ...
break;
}

This is not limited to state machines. Take this example, adapted from an SO answer by Dan Larsen:

switch (color) {
default:
printf("No color selected so defaulting to ");
// Fallthrough
case COLOR_RED:
printf("red");
break;
case COLOR_BLUE:
printf("blue");
break;
case COLOR_GREEN:
printf("green");
break;
}

If color is red, blue or green, the code will print just the color. Otherwise (if color is e.g. yellow) the code will print "No color selected so defaulting to red."

Though maybe it would be clearer to move the default case closer to the bottom (unless you want to sort the cases in logical order):

switch (color) {
case COLOR_BLUE:
printf("blue");
break;
case COLOR_GREEN:
printf("green");
break;
default:
printf("No color selected so defaulting to ");
// Fallthrough
case COLOR_RED:
printf("red");
break;
}

This is now similar to supercat's "alternative arrangement [of the state machine where] an invalid state should not reset the machine but should be readily identifiable as an invalid state":

switch (widget_state) {
case WIDGET_IDLE:
widget_ready = 0;
widget_hardware_off();
break;
case WIDGET_START:
// ...
break;
case WIDGET_WHATEVER:
// ...
break;
default:
widget_state = WIDGET_INVALID_STATE;
/* Fall through */
case WIDGET_INVALID_STATE:
widget_ready = 0;
widget_hardware_off();
// ... do whatever else is necessary to establish a "safe" condition
}

Preventing accidental fallthroughs to the default case

If the default case is the first one, it's impossible for other cases to accidentally fall through to the default case.

This idea is from an SO answer by JaredPar. He continues to clarify: "This means default will run if, and only if, the value matches no case statements in the switch block."

Using a linter would also help you catch accidental fallthroughs, so I don't know whether the confusion caused by placing the default case to the top outweighs the benefits. (To be fair, JaredPar also writes: "Note I'm not saying I agree with it, this is simply the logic others have presented to me in the past.")

So, should you use non-last default cases?

Unless you have a good reason to do otherwise, it's probably safest to put the default case to the end. This way anyone familiar with switch statements can more easily understand the structure of your switch statement.

But if you have a good reason, go ahead. Just be prepared that your code might be initially rather confusing to other developers.

What about the Unity code?

I would put the mindboggling code sent to me by my friend to the Cases in logical order category. I think it also somewhat belongs to the Flow control with fallthroughs category.

After some thought, it's actually logical to present the game states (switch cases) in the order they have been presented: "waiting to start," "playing" and "dead." That's the natural flow of the game states. The initial game state is "waiting to start," so the most logical position for the default case is at the beginning.

So, in the end, the code was actually quite clever!