Why are cases in a switch statement not isolated? [duplicate]

Question

As far as I can tell in all C-like languages something like this:

switch(variable) {
    case 'a':
        printf("Hello ");
    case 'b':
        printf("World!");
    case 'c':
        printf("\n");
}

Will print "Hello World!" if variable == 'a'. I'm curious about the reasoning behind this behaviour as it seems to me automatically inserting breaks at the end of every case would make its behaviour a lot more consistent with expectations and a lot less error-prone.

For example: "In case your die rolls a 6, draw another card" is an action that has to be done in a single case, which to me the case keyword implies.

Answer 1

To write the equivalent of a switch statement in assembly you'd need to add jumps at the end of each branch to prevent execution from continuing to the next branch. C was designed to have a fairly straightforward mapping to machine code, and the pattern would've been familiar to assembly programmers at the time.

Its descendants likely kept it out of historical inertia, which should not be underestimated. For example, the designers of C# clearly felt accidental fall-through is a major source of bugs and is the uncommon case, so you need to explicitly fall through if you want it, but they still kept the syntax of adding a break at the end!

Answer 2

That is because you can't assume that logic and program flow should stop after a single match. It is a good design for a multiple-match type of conditional test.

Take this for example:

switch (shape)
{
    case Square:
        printf("I am a square\r\n");
    case Rectangle:
        printf("I am a rectangle\r\n");
        break;
    case Circle:
        printf("I am a circle\r\n");
        break;
}

If we didn't have the fall through behavior here, we'd have to be redundant with printf("I am a rectangle\r\n"); in order to have that code execute for both a square and a rectangle.

Answer 3

Case statement fall through is rarely needed, but it does sometimes happen. (I have heard that C was originally meant to program telephone switchboards, in which this need arises often, but I was unable to confirm it.) But the truth is that even in modern software, the need for a case statement fall through does arise every once in a rare while.

If the C language did not allow case statement fall-through, then workarounds would be necessary, but the line of thinking back in 1972 was to avoid wasting clock cycles. By allowing case statement fall-through, programmers could have it both ways: you could either fall through or break, whichever was pertinent to your needs, on a case per case basis.

The primary focus of the C language originally was to be nice to the machine, not nice to the programmers. The original C was lean, mean, and very unforgiving.

The idea behind preventing case statement fall through is a very modern concept, and it is all about protecting programmers from themselves. Very few people were thinking along such lines in AT&T Bell Labs back in 1972, when C was invented.

Answer 4

See Stack Overflow.

A summation of the popular arguments presented there is that:

1. A switch statement can provide fallthrough while still allowing you to have a choice about it with the break statment.
2. It was a mistake in the design of C that ended up becoming the standard for case statments.