Preprocessor, const, static, and enum

What runs before the compiler

• Before the compiler sees your code, the preprocessor handles every line that starts with #.
• It pastes in headers, replaces macros, and removes comments — a simple text step.
• You have used it already: #include <stdio.h> is a preprocessor command.

#include and #define

• #include <stdio.h> pastes the contents of a header so you can use its functions.
• #define PI 3.14159 makes PI a macro: the preprocessor replaces every PI with that text.
• Macros have no type and no semicolon — they are pure text replacement.

const vs #define

• const double pi = 3.14159; is a real variable that cannot be changed. It has a type, so the compiler checks it.
• #define PI 3.14159 is just text replacement, done before compiling.
• Modern C usually prefers const for values, but #define is still common for constants and flags.

static and enum

• A static variable inside a function keeps its value between calls — it is set up once, not reset every time the function runs.
• An enum gives names to a set of related int values: enum Suit { HEARTS, DIAMONDS, CLUBS, SPADES }; (here HEARTS is 0).
• Both make code clearer: static for memory that persists, enum for named choices.

#include <stdio.h>

#define GREETING "Hello"
enum Color { RED, GREEN, BLUE };

int main(void) {
    printf("%s\n", GREETING);   // Hello
    enum Color c = GREEN;
    printf("%d\n", c);          // 1
    return 0;
}

Common mistakes

• #define does plain text replacement — wrap macro bodies in parentheses.
• const cannot change; a static local keeps its value between calls.

Now you try

• Use the given #define and enum in the starters.
• A static local variable remembers its value across calls. Do not write a main — the checker provides one.