📘 Section 6 — Pointers

Understand memory addresses, pointer variables, and how to use pointers with arrays and functions.

🎯 What You’ll Learn

What pointers are and why they matter
How to declare and use pointer variables
Pointer arithmetic with arrays
Passing pointers to functions

1. Some Pointers Usage

Dynamic memory allocation
Passing parameters to functions
Arrays and strings
File handling
Pointer Arithmetic and Complex Data Structures

2. What Is a Pointer?

A pointer is a variable that stores the memory address of another variable.

#include <stdio.h>

int main() {
    int x = 10;
    int *p = &x;
    *p = 20;       // changes x to 20 through the pointer
    return 0;
}

How the Pointer Looks in Memory

After int *p = &x; and before *p = 20;, assume the computer stores x at address 1000, and stores the pointer p at address 2000.

Address 1000

p stores this address

Address 2000

1000

The value inside p is 1000, so p stores the address of x.

So:

Expression	Meaning	Value in this example
`x`	Value stored in `x`	`10`
`&x`	Address of `x`	`1000`
`p`	Value stored in pointer `p`	`1000`
`*p`	Go to address stored in `p` and get the value there	`10`
`&p`	Address of the pointer variable itself	`2000`

A pointer is also a variable, so it has its own address. The special thing about a pointer is that the value inside it is an address.

When we write *p = 20;, C goes to the address stored inside p (1000) and changes the value of x from 10 to 20.

Printing Pointer Info

int num = 42;
int *ptr = &num;

printf("Value of num: %d\n", num);        // Output the value of 'num'
printf("Address of num: %p\n", &num);     // Output the address of 'num'
printf("Value stored in ptr: %p\n", ptr); // Output the value stored in the pointer
printf("Value pointed to by ptr: %d\n", *ptr); // Output the value pointed to by the pointer

3. Pointers and Arrays

An array name acts as a pointer to its first element:

int arr[] = {1, 2, 3, 4, 5};
int *ptr = arr;

for (int i = 0; i < 5; i++) {
    printf("%d ", *(ptr + i));
}

Expression	Equivalent	Value
`*(ptr + 0)`	`arr[0]`	1
`*(ptr + 1)`	`arr[1]`	2
`*(ptr + 2)`	`arr[2]`	3

💡 ptr + i moves the pointer forward by i elements (not i bytes — C handles the size automatically).

🧪 Exercises

Exercise 1: Print Array Using a Pointer

Task: Write a program in C to store n elements in an array and print the elements using a pointer.

💡 Hint

Declare a pointer to the array: int *ptr = arr;
Use *ptr to access the current element, then move with ptr++

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int n;
    printf("Enter number of elements: ");
    scanf("%d", &n);
    int arr[n];

    printf("Enter %d elements: ", n);
    for (int i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }

    int *ptr = arr;
    printf("Array elements: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", *ptr);
        ptr++;
    }

    return 0;
}

Exercise 2: Sum & Average with Pointers

Task: Create a program that uses pointers to find the sum and average of elements in an array.

💡 Hint

Traverse the array using pointer arithmetic
Sum all elements, then divide by n for the average
Use *ptr to read the current element, then ptr++ to move to the next element
Use float or double for the average

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int n;
    printf("Enter number of elements: ");
    scanf("%d", &n);
    int arr[n];

    printf("Enter %d elements: ", n);
    for (int i = 0; i < n; i++) {
        scanf("%d", &arr[i]);
    }

    int *ptr = arr;
    int sum = 0;
    for (int i = 0; i < n; i++) {
        sum += *ptr;
        ptr++;
    }

    printf("Sum = %d\n", sum);
    printf("Average = %.2f\n", (float)sum / n);

    return 0;
}

Exercise 3: Copy Array Using Pointers

Task: Write a program that uses pointers to copy one array to another.

💡 Hint

Create two arrays and two pointers
Loop through and assign *ptr2 = *ptr1, then increment both pointers

🟢 Click to Show Solution

#include <stdio.h>

int main() {
    int n;
    printf("Enter number of elements: ");
    scanf("%d", &n);

    int arr1[n], arr2[n];
    printf("Enter %d elements: ", n);
    int *ptr1 = arr1;
    int *ptr2 = arr2;

    for (int i = 0; i < n; i++) {
        scanf("%d", ptr1);
        ptr1++;
    }

    ptr1 = arr1;
    for (int i = 0; i < n; i++) {
        *ptr2 = *ptr1;
        ptr1++;
        ptr2++;
    }

    printf("Copied array: ");
    for (int i = 0; i < n; i++) {
        printf("%d ", arr2[i]);
    }

    return 0;
}

Exercise 4: Swap Two Numbers Using Pointers

Task: Write a function that takes two integer pointers and swaps the values of the numbers.

💡 Hint

The function signature should be void swap(int *a, int *b)
Use a temporary variable to hold one value during the swap
Call with swap(&x, &y) from main

🟢 Click to Show Solution

#include <stdio.h>

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

int main() {
    int x = 5;
    int y = 3;
    printf("before swap: x= %d, y= %d\n", x, y);

    swap(&x, &y);

    printf("after swap: x= %d, y= %d\n", x, y);
    return 0;
}

Output:

before swap: x= 5, y= 3
after swap: x= 3, y= 5

💡 This is call by reference — the function modifies the original variables because it has their addresses, not copies.

Exercise 5: Find Min & Max Using Pointers

Task: Write a function that takes an integer array and its size as parameters and updates two integer pointers to point to the maximum and minimum values in the array.

💡 Hint

Function signature: void maxAndMin(int arr[], int size, int *max, int *min)
Initialize *max and *min to the first element
Loop through and update using *max and *min

🟢 Click to Show Solution

#include <stdio.h>

void maxAndMin(int arr[], int size, int *max, int *min) {
    *max = *min = arr[0];
    for (int i = 1; i < size; i++) {
        if (arr[i] > *max) {
            *max = arr[i];
        } else if (arr[i] < *min) {
            *min = arr[i];
        }
    }
}

int main() {
    int arr[] = {2, 1, 3, 4, 2, 2};
    int size = sizeof(arr) / sizeof(arr[0]);
    int max, min;

    maxAndMin(arr, size, &max, &min);

    printf("Max= %d - Min= %d\n", max, min);

    return 0;
}

Output:

Max= 4 - Min= 1

💡 By passing &max and &min, the function can write results directly into main’s variables — no return value needed for multiple outputs.

📋 Quick Reference

Concept	Syntax
Declare pointer	`int *ptr;`
Get address	`ptr = &variable;`
Dereference (get value)	`*ptr`
Pointer arithmetic	`*(ptr + i)` is same as `arr[i]`
Pass by reference	`void func(int *p)` called with `func(&x)`

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