Call by Value vs Call by Reference in C

In this method, the actual value of the variable is copied and passed to the function.

• The function creates its own temporary variables (formal parameters) to hold this copied data.
• Rule: If you make any changes to these variables inside the function, the original variables in the main() function are NOT affected.
• It is like giving someone a photocopy of your notes. If they scribble on the photocopy, your original notes remain clean.

Example use case: Simple mathematical calculations where you just need the numbers to compute a result.

In this method, instead of passing the data, you pass the memory address (pointer) of the variable to the function.

• The function does not make a copy. It goes directly to the memory address of the original variable.
• Rule: Any changes made inside the function are permanently reflected in the original variables back in main().
• It is like giving someone the actual notebook. If they scribble on it, your original notes are permanently changed.

Example use case: Swapping the values of two variables or modifying large arrays without copying them.

To swap two numbers successfully, you MUST use Call by Reference.

Incorrect (Call by Value):

void swap(int a, int b) {
    int temp = a;
    a = b;
    b = temp;
}
// The swap only happens inside the function. Original variables in main() don't change.

Correct (Call by Reference):

void swap(int *a, int *b) { // Takes pointers (addresses)
    int temp = *a;
    *a = *b;
    *b = temp;
}
// Called in main() using the '&' (address of) operator: swap(&x, &y);
// Original variables x and y are successfully swapped.