20250624 17:00 Functions

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Scope Rules: Who Knows What?

Imagine your variables are secret agents 

main() {
  int i = 20;
  display(i);
}

display(int j) {
  int k = 35;
  printf("%d", j);
  printf("%d", k);
}

• i is known only inside main(). It’s local.

• k is known only inside display(). Also local.

If display() wants i, main must share it (pass it like a gift ).
If main() wants k, display() must return it (send it back ).

Moral of the story: What happens in a function, stays in that function. 

Calling Convention: Left or Right?

When you call a function like this:

fun(a, b, c, d);

C says, “Let’s start from the right!” 
Yep, C passes arguments right to left. 

Example:

int a = 1;
printf("%d %d %d", a, ++a, a++);

Looks like it should print 1 2 3, right?

Nope! It prints 3 3 1
Why? Because of:

• a++ passes 1 (then a = 2)

• ++a makes it 3 and passes 3

• Last a is now 3

Confusing? Yep 
Just don’t do crazy stuff in printf() — it's not a magic show 

Library Functions & Prototypes

When you use cool built-in functions like:

clrscr();
gotoxy(10, 20);
ch = getch();

These come from secret header files like conio.h.
Each .h file = a box of tools (aka prototypes).

• stdio.h → for input/output tools

• math.h → for math nerd stuff 

• conio.h → for console tricks 

Want to look smart? Peek inside .h files!
You’ll see stuff like:

void clrscr();
int getch();

If you don’t match the right tools with the right format, C will still compile but it might print garbage values 

Advanced Function Features

Time to level up! Here come the cool moves:

1. Function Prototypes

C Thinks Every Function Returns int

So if your function gives a float, you must tell C!

Bad:

square(float x) { return x * x; }

C says: “Hmm... must be int!” → Gives wrong result like 2 instead of 2.25 

Good:

float square(float x) { return x * x; }

Also tell C before calling it:

float square(float);

Now C knows it's dealing with floats — no more math mistakes!

2. Returning Nothing? Use void

Want a function that just talks and doesn’t return anything?

void gospel() {
  printf("Viruses are electronic bandits...\n");
  printf("They eat nuggets of information...\n");
  printf("And bite your bytes!\n");
}

void means: "Don't expect anything back."

L;DR (Too Long; Didn’t Read)

• Variables live only inside the function where they’re declared.

• C passes arguments from right to left.

• Use .h files for library functions.

• Want to return a float? Declare it properly!

• No return value? Use void.