Lesson 24: Functions (Basics)

f(x) = 2x + 1

f(3) → substitute 3 for x → 7
f(10) → substitute 10 for x → 21

① Arguments and return values have types

In math, f(x) = 2x + 1 works whether x is an integer or a real number. In C, you declare the type of every input and output.
int f(int x) means "takes an int, returns an int." For decimal values, you'd write double f(double x) as a separate function.

② Functions can have side effects — some return nothing

A math function simply maps input to output. A C function can also print to the screen, modify files, or change variables — these are called side effects.

void hello(void) {        // no return (void)
    printf("Hello!\n");   // side effect: output to screen
}

void means "this function returns no value" — a concept with no direct equivalent in math.

③ Same input can give different outputs

In math, f(3) is always 7. A C function, however, can return a different value each time it's called:

int r = rand();        // random number (different each call)
int n = getchar();     // user keyboard input
time_t t = time(NULL); // current clock time

These all depend on external state (the random seed, stdin, or the system clock).

④ Arguments are copied (pass by value)

In math, you just "pass x." In C, only the value is copied into the function, so changing the parameter inside the function doesn't affect the caller's variable.

void change(int x) { x = 100; }  // sets local x to 100
int main(void) {
    int a = 5;
    change(a);
    printf("%d\n", a); // -> 5 (unchanged!)
}

To actually modify the caller's variable, you need pointers (covered in a later lesson).

⑤ Functions can take no arguments

Math rarely uses zero-argument functions, but they're common in C.

int get_year(void) {   // void = no parameters
    return 2026;
}

They're useful for fetching external data or returning a fixed value.