19.7 switch Statement

The switch statement selects code to run according to the value of an expression. The expression, in parentheses, follows the keyword switch. After that come all the cases to select among, inside braces. It looks like this:

switch (selector)
  {
    cases…
  }

A case can look like this:

case value:
  statements
  break;

which means “come here if selector happens to have the value value,” or like this (a GNU C extension):

case rangestart ... rangeend:
  statements
  break;

which means “come here if selector happens to have a value between rangestart and rangeend (inclusive).” See Case Ranges.

The values in case labels must reduce to integer constants. They can use arithmetic, and enum constants, but they cannot refer to data in memory, because they have to be computed at compile time. It is an error if two case labels specify the same value, or ranges that overlap, or if one is a range and the other is a value in that range.

You can also define a default case to handle “any other value,” like this:

default:
  statements
  break;

If the switch statement has no default: label, then it does nothing when the value matches none of the cases.

The brace-group inside the switch statement is a block, and you can declare variables with that scope just as in any other block (see Blocks). However, initializers in these declarations won’t necessarily be executed every time the switch statement runs, so it is best to avoid giving them initializers.

break; inside a switch statement exits immediately from the switch statement. See break Statement.

If there is no break; at the end of the code for a case, execution continues into the code for the following case. This happens more often by mistake than intentionally, but since this feature is used in real code, we cannot eliminate it.

Warning: When one case is intended to fall through to the next, write a comment like ‘falls through’ to say it’s intentional. That way, other programmers won’t assume it was an error and “fix” it erroneously.

Consecutive case statements could, pedantically, be considered an instance of falling through, but we don’t consider or treat them that way because they won’t confuse anyone.