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#### 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.