# elif in c

Processor directive #elif is part of #if directive. It will execute only if it’s just previous constant expression of #if or #elif is zero.  

Syntax:

#if <Constant_expression>

    ------------------

    ------------------

#elif <Constant_expression>

    ------------------

    ------------------

………………………….

………………………..

#elif <Constant_expression>

    ------------------

    ------------------

#else

    ------------------

    ------------------

#endif

Example 1:

#include<stdio.h>

int main(){

    int num=11;

    #if(num>0)

         ++num;

    #elif(num==0)

         --num;

    #else

         num=0;

    #endif

    printf("%d",num);

return 0;

}

Output: Compilation error

Explanation: we cannot use logical operator > with undefined macro constant num.

Note: Preprocessor directive #if will treat num as macro constant not as integer variable since it execute just before the actual c code.

Example 2:

#include<stdio.h>

int main(){

    #if(!5>=5)

         int a=5;

    #elif -1

         int a=10;

    #else

         int a=15;

    #endif

    printf("%d",a);

return 0;

}

Output: 10

Explanation: Consider on the expression:  !5 >= 5

= 0 >= 5

= 0

So #if is false. Now it will execute first #ielif directive. Since -1 is non-zero number. So condition is true so #else directive will not execute.

Its intermediate file will look like:

int main(){

    int a=10;

    printf("%d",a);

return 0;

}

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