#if directive in c

There are total six conditional compilation directives. There are:

(a)#if

(b)#elif

(c)#else

(d)#endif

(e)ifdef

(f)ifndef

#if directive :

    It is conditional compilation directive. That is if condition is true then it will compile the c programming code otherwise it will not compile the c code.

Syntax 1:

#if <Constant_expression>

    -------------

    -------------

#endif

If constant expression will return 0 then condition will ture if it will return any non zero number condition will true.

Example:

#include<stdio.h>

#if 0

int main(){

    printf("HELLO WORLD");

return 0;

}

#endif

Output: Run time error, undefined symbol _main

Explanation: Due to zero as a constant expression in #if condition will false. So c code inside the #if condition will not execute. As we know without any main function we cannot execute any code.

We can also write #else in the #if directive. #else directive will only execute if condition is false.

Syntax 2:

#if <Constant_expression>

    -----------------

    -----------------

#else

    -----------------

    -----------------

#endif

Example:

#include<stdio.h>

#if(10)

int main(){

    printf("errorandexception.blogspot.com");

return 0;

}

#else

int main(){

    We can write any thing

    5= a int;

    if(for(while(1);;));

    ++5;

    10=24;

return 0;

}

#endif

Output: errorandexception.blogspot.com

Explanation: 10 is non zero integer constant. So #if condition is true.

Example:

#include<stdio.h>

#if -2

int main(){

    printf("HELLO WORLD");

return 0;

}

#else

int main(){

    printf("errorandexception.blogspot.com");

return 0;

}

#endif

Output: HELLO WORLD

Explanation: -2 is non zero number so #if condition is true.

Note: Consonant expression in #if condition should not include any c programming variable since all preprocessor directives execute just before the actual c code.

For example:

#include<stdio.h>

int main(){

int var = 5;

    #if var

        printf("errorandexception.blogspot.com");

    #else

         printf("cquestionbank.blogspot.com");

    #endif

return 0;

}

Output: cquestionbank.blogspot.com

Explanation: Directive #if will not think expression constant var as integer variable and also it will not throw an error. Then what is var for directive #if?

Directive #if will treat var as underfed macro constant. In c any underfed macro constant return zero so #else directive will execute. So proper way of above c code is:

#include<stdio.h>

#define var 10

int main(){

    #if var

        printf("errorandexception.blogspot.com");

    #else

         printf("cquestionbank.blogspot.com");

    #endif

return 0;

}

Output: errorandexception.blogspot.com

Explanation: Macro constant var will be replaced 10. Since 10 is non-zero number so #if part will execute.

Note: Constant expression in #if directive cannot be string constant. It can be character constant which returns its ASCII value to directive.

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