In C Language, the "#undef" directive is used to "undefine" a previously defined macro.
This allows us to remove a macro definition within the scope of our program.
It's useful when we need to change the behaviour of macros in different parts of our code.
#undef identifier
#undef syntax is used to declared `undef` macro in code, where "identifier" is the name of the macro we want to remove.
#include <stdio.h>
#define PI 3.14159
int main() {
printf("Value of PI: %f\n", PI);
#undef PI
#ifdef PI
printf("Value of PI after undefining: %f\n", PI);
#else
printf("PI is not defined anymore.\n");
#endif
return 0;
}
Value of PI: 3.141590 PI is not defined anymore.
we first define a macro "PI" with the value 3.14159. Then, we print its value. After that, we "#undef" the macro PI.
Finally, we check if "PI" is still defined. Since we have undefined it, the "#ifdef" block won't execute, and it will print "PI is not defined anymore."
"#undef" can be useful in situations where we want to remove or redefine a macro within a specific scope of your program.
However, use it with caution, as removing a macro may have unintended consequences on the behaviour of your code.
#include <stdio.h>
#define PI 3.14159
int main() {
printf("Value of PI: %f\n", PI);
#undef PI
#define PI 3.14
printf("Value of PI after redefining: %f\n", PI);
return 0;
}
Value of PI: 3.141590 Value of PI after redefining: 3.140000
we first define the macro PI with the value 3.14159.
Then, we "#undefine" PI and "redefine" it with a different value "3.14".
This demonstrates how we can redefine a macro after removing its previous definition.