📌 Using Pointers with Functions in C

📌 Pointers with Functions in C

In C programming, pointers can be used to pass variables to functions by reference. This allows the function to directly modify the original variable, unlike the default call by value method.

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🎯 Why Use Pointers with Functions?

• To allow a function to modify the actual variables passed as arguments.
• To return multiple values from a function.
• To pass large data structures (arrays, structs) efficiently.

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📘 Function Call by Value (Default)

Passing variables without pointers does not change the original values.

#include <stdio.h>

void change(int a) {
    a = 100;
}

int main() {
    int x = 10;
    change(x);
    printf("x = %d\n", x);  // x is still 10
    return 0;
}

❌ Output:

x = 10

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✅ Function Call by Reference (Using Pointers)

Passing the address of a variable allows the function to modify the original.

#include <stdio.h>

void change(int *a) {
    *a = 100;
}

int main() {
    int x = 10;
    change(&x);
    printf("x = %d\n", x);  // x is changed to 100
    return 0;
}

✔ Output:

x = 100

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🔄 Swap Two Numbers Using Pointers

#include <stdio.h>

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

int main() {
    int x = 5, y = 10;
    swap(&x, &y);
    printf("x = %d, y = %d\n", x, y); // x = 10, y = 5
    return 0;
}

✔ Output:

x = 10, y = 5

Explanation: We passed addresses to swap(), allowing direct modification.

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📌 Passing Arrays to Functions

Arrays are always passed as pointers. The function gets access to the original array.

#include <stdio.h>

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

int main() {
    int numbers[] = {1, 2, 3, 4, 5};
    printArray(numbers, 5);
    return 0;
}

🧪 Output:

1 2 3 4 5

Note: You can also use arr[] as the parameter. It's just syntax sugar for a pointer.

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🧩 Practice Exercises

1. Write a function that increments a number using a pointer.
2. Write a function that finds the maximum of two numbers using pointers.
3. Create a function to reverse an array using pointer notation.
4. Use a function to calculate the sum of an array passed by pointer.

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🧠 Summary

• Call by value: Passes a copy, original unchanged.
• Call by reference (using pointers): Allows functions to modify the original variable.
• Arrays are passed as pointers by default.

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