C #ifndef

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Description

In the C Programming Language, the #ifndef directive allows for conditional compilation. The preprocessor determines if the provided macro does not exist before including the subsequent code in the compilation process.

The #ifndef preprocessor directive checks if macro is not defined by #define. If yes, it executes the code otherwise #else code is executed, if present.

Syntax:





#ifndef MACRO  
//code  
#endif


Syntax with #else: 


#ifndef MACRO  
//successful code  
#else  
//else code  
#endif


Note

The #ifndef directive must be closed by an #endif directive.

C #ifndef example

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




#include <stdio.h>
#include <conio.h>
#define INPUT  
void main() {  
int a=0;  
#ifndef INPUT  
a=2;  
#else  
printf("Enter a:");  
scanf("%d", &a);  
#endif         
printf("Value of a: %d\n", a);  
getch();  
}


Output:

Enter a:5

Value of a: 5

But, if you don't define INPUT, it will execute the code of #ifndef.




#include <stdio.h>
#include <conio.h>
void main() {  
int a=0;  
#ifndef INPUT  
a=2;  
#else  
printf("Enter a:");  
scanf("%d", &a);  
#endif         
printf("Value of a: %d\n", a);  
getch();  
}


Output:

Value of a: 2

Example

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




/* Example using #ifndef directive by C programming */

#include <stdio.h>

#define YEARS_OLD 15
#ifndef YEARS_OLD
#define YEARS_OLD 10
#endif

int main()
{
   printf("C programming is over %d years old.\n", YEARS_OLD);

   return 0;
}


In this example, if the macro YEARS_OLD is not defined before the #ifndef directive is encountered, it will be defined with a value of 10.

Here is the output of the executable program:

C programming is over 15 years old.

If you remove the line #define YEARS_OLD 15, you will see the following output from the executable program:

C programming is over 10 years old.




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