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What is #else directive in C?

Muhammad Ashir

The #else preprocessor directive is used with the #if, #ifdef, #ifndef directives. The preprocessor includes the code in the #else directive if the condition in the #if, #ifdef or #ifndef is not met.

The #else directive must be closed with a #endif directive.

Syntax

#else
/* 
your code here
*/
#endif

Code

#include<stdio.h>

int main() {
  #if 1 < 0
    printf("The code in #if directive is run\n");
  #else
    printf("The code in #else directive is run\n");
  #endif
  return 0;
}

The code above demonstrates how to use the #else directive. The conditional expression in the #if directive evaluates to either zero or a non-zero value. If it evaluates to a non-zero value, the preprocessor selects the code in the #if block. If the expression evaluates to 0, the code in the #else directive is run.

The preprocessor passes the selected code block to the compiler for compilation. Together with the #if, #ifdef and #ifndef directives, the #else directive can be used for conditionally compiling certain portions of the source code.

Try changing the expression in the following code to see how conditional compilation works:

#if 1 < 0

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