Pointers and Functions

🔍 Why Use Pointers with Functions?

C passes function arguments by value, which means:

• The function gets a copy of the value.

• Changes inside the function do not affect the original variable.

➡️ But with pointers, we can:

• Pass addresses, not values.

• Modify variables directly from within the function.

🧾 Function Argument Types in C

📦 Example 1: Pass by Value vs Pass by Pointer

void updateByValue(int x) {
    x = x + 10;
}

void updateByPointer(int *p) {
    *p = *p + 10;
}

int main() {
    int num = 5;

    updateByValue(num);
    printf("After updateByValue: %d\\n", num); // 5

    updateByPointer(&num);
    printf("After updateByPointer: %d\\n", num); // 15

    return 0;
}

📌 Explanation:

• updateByValue(num) → num is copied, so original not changed.

• updateByPointer(&num) → Address is passed, so modification reflects in original variable.

📥 Syntax for Function with Pointer Parameter

void functionName(type *pointerName) {
    // use *pointerName to access or modify the value
}

📦 Example 2: Swapping Two Numbers Using Pointers

void swap(int *a, int *b) {
    int temp = *a;
    *a = *b;
    *b = temp;
}

int main() {
    int x = 10, y = 20;
    swap(&x, &y);
    printf("x = %d, y = %d\\n", x, y);  // Output: x = 20, y = 10
    return 0;
}

🧠 *a and *b refer to the actual memory where x and y are stored. Changes reflect outside.

🧩 Arrays Passed as Pointers

Arrays are always passed as pointers to the first element.

void printArray(int *arr, int size) {
    for(int i = 0; i < size; i++) {
        printf("%d ", arr[i]);
    }
}

📌 Can also be declared like:

void printArray(int arr[], int size)

But under the hood, both versions treat arr as a pointer.

🧠 Function Pointer: Functions as Variables!

You can also have a pointer to a function, just like a pointer to a variable.

void greet() {
    printf("Hello World!\\n");
}

int main() {
    void (*funcPtr)();  // Function pointer declaration
    funcPtr = greet;    // Assign address of function
    funcPtr();          // Call the function through pointer

    return 0;
}

📌 Very useful in:

• Callback functions

• Menu-driven programs

• Dynamic function calls

🧪 Practical Example: Dynamic Calculator Using Function Pointers

int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }

int main() {
    int (*operation)(int, int);
    int choice = 1, x = 5, y = 3;
    if (choice == 1)
        operation = add;
    else
        operation = subtract;
    printf("Result: %d\\n", operation(x, y)); // Result: 8
    return 0;
}

🛠️ Real World Use

• 🔧 Swapping or updating values in-place

• 🧠 Dynamic programming where memory use matters

• 🛠️ Callback mechanisms in drivers/OS

• 📚 Pass-by-reference simulation