20250625 10:00 Pointers

                         

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Call by Value vs Call by Reference (With Pointer Magic!)

Call by Value – “Copy Paste”

Whenever you call a function like this:

sum = calsum(a, b, c);

You're actually saying:
"Hey function! Take copies of my values and play with them. But don’t touch the originals!"

Example:

main() {
  int a = 10, b = 20;
  swapv(a, b);
  printf("a = %d, b = %d", a, b);
}

swapv(int x, int y) {
  int t = x;
  x = y;
  y = t;
  printf("x = %d, y = %d", x, y);
}

Output:

x = 20, y = 10
a = 10, b = 20

Call by Reference – “Handle with Care (and Address)”

Now instead of sending copies, you send the actual address where the data lives!

main() {
  int a = 10, b = 20;
  swapr(&a, &b);
  printf("a = %d, b = %d", a, b);
}

swapr(int *x, int *y) {
  int t = *x;
  *x = *y;
  *y = t;
}

Output:

a = 20, b = 10

Success! Real values got swapped!
Why? Because *x means "value at the address" — it’s like having the home key instead of a photo. 

Meet the Pointer – C’s Favorite Trick

A pointer is like a post-it note that says:

"Hey! I know where the real data lives!"

int i = 3;
int *j;   // j will point to an int
j = &i;   // j now holds the address of i

So...

• i is the real value (3)

• &i is the address of i (say 65524)

• j holds that address

• *j means: go to that address and get the value = 3

So *j == i. Magic? Nah, just pointers! 

Pointer in a Pointer? YES!

Not this pointer!

Want more pointer madness? Let’s go full Inception 

int i = 3;
int *j = &i;
int **k = &j;

• i is 3

• j holds address of i

• k holds address of j

So:

• *k → value of j → address of i

• **k → value at address stored in j → value of i → 3

It’s like asking your friend to ask another friend where your secret cookie stash is. 

Mixing Value + Reference: Return More Than One Thing!

C normally lets you return only 1 value from a function. 
But what if we want 2 or more? Use pointers!

main() {
  int r = 5;
  float area, peri;

  areaperi(r, &area, &peri);
  printf("Area = %f, Perimeter = %f", area, peri);
}

areaperi(int r, float *a, float *p) {
  *a = 3.14 * r * r;
  *p = 2 * 3.14 * r;
}

Boom! One function, two results returned using addresses!
It's like sending two containers and asking the function to fill them.