The #elif Directive

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The #elif directive

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 the previous input to be abbreviated, as the following example shows.

#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 “#if”and “#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” and “#endif” group. Then the text after each #elif 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.

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