C #if

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Description

In the C Programming Language, the #if directive allows for conditional compilation. The preprocessor evaluates an expression provided with the #if directive to determine if the subsequent code should be included in the compilation process.

The #if preprocessor directive evaluates the expression or condition. If condition is true, it executes the code otherwise #elseif or #else or #endif code is executed.

Syntax:




#if expression  
//code  
#endif


Syntax with #else: 


#if expression  
//if code  
#else  
//else code  
#endif


Syntax with #elif and #else: 


#if expression  
//if code  
#elif expression  
//elif code  
#else  
//else code  
#endif


Note

  • The #if directive must be closed by an #endif directive.

C #if example

Let's see a simple example to use #if preprocessor directive.




#include <stdio.h>
#include <conio.h>
#define NUMBER 0  
void main() {  
#if (NUMBER==0)  
printf("Value of Number is: %d",NUMBER);  
#endif         
getch();  
}


Output:

Value of Number is: 0

Let's see another example to understand the #if directive clearly.




#include <stdio.h>
#include <conio.h>
#define NUMBER 1  
void main() {  
clrscr();  
#if (NUMBER==0)  
printf("1 Value of Number is: %d",NUMBER);  
#endif  
  
#if (NUMBER==1)  
printf("2 Value of Number is: %d",NUMBER);  
#endif  
getch();  
}


Output:

2 Value of Number is: 1

Example

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




/* Example using #if directive */

#include <stdio.h>

#define WINDOWS 1

int main()
{
   printf("C programming is a great ");

   #if WINDOWS
   printf("Windows ");
   #endif

   printf("resource.\n");

   return 0;
}


Here is the output of the executable program:

C programming is a great Windows resource.




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