4.2.5 Elif

One common case of nested conditionals is used to check for more than two possible alternatives. For example, you might have

#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 follows:

#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 a conditional group and subdivides it; it does not require a matching ‘#endif’ of its own. 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 conditional group. Then the text after each ‘#elif’ is processed only if the ‘#elif’ condition succeeds after the original ‘#if’ and all previous ‘#elif’ directives within it have failed.

‘#else’ is allowed after any number of ‘#elif’ directives, but ‘#elif’ may not follow ‘#else’.