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C Language: #elif Directive

This C tutorial explains how to use the #elif preprocessor directive in the C language.

Description

In the C Programming Language, the #elif provides an alternate action when used with the #if, #ifdef, or #ifndef directives. The preprocessor will include the C source code that follows the #elif statement when the condition of the preceding #if, #ifdef or #ifndef directive evaluates to false and the #elif condition evaluates to true.

The #elif directive can be thought of as #else if.

Syntax

The syntax for the #elif directive in the C language is:

#elif conditional_expression
conditional_expression
Expression that must evaluate to true for the preprocessor to include the C source code into the compiled application.

Note

Example

The following example shows how to use the #elif directive in the C language:

/* Example using #elif directive by TechOnTheNet.com */

#include <stdio.h>

#define YEARS_OLD 12

int main()
{
   #if YEARS_OLD <= 10
   printf("TechOnTheNet is a great resource.\n");
   #elif YEARS_OLD > 10
   printf("TechOnTheNet is over %d years old.\n", YEARS_OLD);
   #endif

   return 0;
}

In this example, YEARS_OLD has a value of 12 so the statement #if YEARS_OLD <=10 evaluates to false. As a result, processing is passed to the #elif YEARS_OLD > 10 statement which evaluates to true. The C source code following the #elif statement is then compiled into the application.

Here is the output of the executable program:

TechOnTheNet is over 12 years old.
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