update the value of a variable which is defined using #define in c

Question

In C language if I define a value using #define eg:

#define STATE 1

Can I update the value of STATE further in the program? If it is possible, tell me how?

Answer 1

The preprocessor is a text-substitution system. In other words, when you use the symbol STATE elsewhere in your program it's replaced by its definition, (1).

For example, if you write:

printf("%d\n", STATE);

The preprocessor replaces STATE with 1 and what the compiler actually "sees" is:

printf("%d\n", 1);

If you were to try to update STATE within C code you'd get errors about assigning to an r-value.

If you want to re-define state within the preprocessor, you could do:

#define STATE 1
#undef STATE
#define STATE 2