Why was the switch statement designed to need a break


Given a simple switch statement

switch (int)
    case 1 :

    case 2 : 

    case 3 : 

The absence of a break statement in case 2, implies that execution will continue inside the code for case 3.
This is not an accident; it was designed that way. Why was this decisions made? What benefit does this provide vs. having an automatic break semantic for the blocks? What was the rationale?

Best Answer

Many answers seem to focus on the ability to fall through as the reason for requiring the break statement.

I believe it was simply a mistake, due largely because when C was designed there was not nearly as much experience with how these constructs would be used.

Peter Van der Linden makes the case in his book "Expert C Programming":

We analyzed the Sun C compiler sources to see how often the default fall through was used. The Sun ANSI C compiler front end has 244 switch statements, each of which has an average of seven cases. Fall through occurs in just 3% of all these cases.

In other words, the normal switch behavior is wrong 97% of the time. It's not just in a compiler - on the contrary, where fall through was used in this analysis it was often for situations that occur more frequently in a compiler than in other software, for instance, when compiling operators that can have either one or two operands:

switch (operator->num_of_operands) {
    case 2: process_operand( operator->operand_2);
              /* FALLTHRU */

    case 1: process_operand( operator->operand_1);

Case fall through is so widely recognized as a defect that there's even a special comment convention, shown above, that tells lint "this is really one of those 3% of cases where fall through was desired."

I think it was a good idea for C# to require an explicit jump statement at the end of each case block (while still allowing multiple case labels to be stacked - as long as there's only a single block of statements). In C# you can still have one case fall through to another - you just have to make the fall thru explicit by jumping to the next case using a goto.

It's too bad Java didn't take the opportunity to break from the C semantics.

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