#undef in c

Explanation of #undef directives in c programming language by examples, questions and answers

Directive #undef is used to undefine any macro constants except global identifiers. It is useful when we want to redefined any macro constants. For example:

#include<stdio.h>

#define ABC 25

#ifdef ABC

#undef ABC

#define ABC 50

#else

#define ABC 100

#endif

int main(){

        printf("%d",ABC);

return 0;

}

Output: 50

Explanation: Since macro constant ABC has already defined. So #ifdef condition is true. Directive #undef will undefined the macro constant ABC but #define will again define. 

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