One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have the following statement.
#if X == 1 . . . #else /* X != 1 */ #if X == 2 . . . #else /* X != 2 */ . . . #endif /* X != 2 */ #endif /* X != 1 */Another conditional directive, #elif, allows this to be abbreviated as in the following example.
#if X == 1 . . . #elif X == 2 . . . #else /* X != 2 and X != 1*/ . . . #endif /* X != 2 and X != 1*/#elif stands for else if. Like #else, it goes in the middle of #ifand #endif pairs, subdividing the pair; #elif does not require a matching #endif. Like #if, the #elif directive includes an expression to be tested.
The text following the #elif is processed only if the original #if condition failed and the #elif condition succeeds. More than one #elif can go in the same #if- #endif group. Then the text after each #e 548 lif is processed only if the #elif condition succeeds after the original #if and any previous #elif directives within it have failed. #else is equivalent to #elif1, and #else is allowed after any number of #elif directives, but #elif may not follow #else. 0