Pointers in C

Pointer Variable in C

A pointer variable is a variable that stores the memory address of another variable.
They are called pointers because they point to a specific location in memory.

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Memory Allocation of Variables

When a variable is declared:

• It is stored in a specific memory location.

• The compiler and operating system automatically decide where it is placed at runtime.

• The programmer does not control the exact memory location.

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Example Declaration

int x = 3;

This tells the C compiler to:

1. Reserve space in memory to store an integer value
   (location/address decided by compiler/OS)

2. Associate the name x with that memory location

3. Store the value 3 in that location

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Accessing Variable Information

1. Size of Allocated Space

printf("%d", sizeof(x));

2. Value Stored in Variable

printf("%d", x);

3. Memory Address of Variable

Use the address-of operator &

printf("%p", &x);

• &x gives the address of variable x

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Declaring a Pointer Variable

Pointers are declared using the indirection operator *

Example

int *p;

• Creates a pointer variable p

• It can store the address of an integer variable

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Assigning Address to Pointer

p = &x;

• Now p contains the address of x

• We say p points to x

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Understanding a Pointer

A pointer has two parts:

1. The pointer itself → holds the address

2. The address → points to the stored value

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Printing Pointer Information

1. Print Address Stored in Pointer

printf("%p", p);

2. Print Value Pointed by Pointer

printf("%d", *p);

• *p means value stored at the address contained in p

Uses of Pointers in C

Pointers can be used to:

• Build faster and more efficient code

• Provide an alternate and efficient way to access data stored in variables and arrays

• Pass arrays and strings conveniently from one function to another

• Return more than one value from a function

• Perform dynamic memory allocation

• Build complex data structures such as linked lists, trees, etc.

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Important Pointer Operators in C

1. Address-of Operator (&)

• Used to get the memory address of a variable

• Returns the location where the variable is stored in memory

Example:

&variable

Returns the address of variable

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2. Dereference Operator (*)

• Used to access the value stored at a pointer’s address

• Also used to declare pointer variables

Example:

*pointer

Accesses the value stored at the memory location pointed to by pointer

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Types of Pointers in C

1. NULL Pointer

• A pointer that does not point to any valid memory location

• Created by assigning NULL to the pointer

• Any pointer type can be assigned NULL

Syntax:

data_type *pointer_name = NULL;

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2. Void Pointer (Generic Pointer)

• Pointer of type void

• Has no associated data type

• Can store address of any type of variable

Syntax:

void *pointer_name;

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3. Constant Pointer

• The address stored in the pointer is constant

• Cannot be modified after initialization

• Always points to the same memory location

Syntax:

data_type * const pointer_name;

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4. Pointer to Constant

• Pointer that points to a constant value

• The value cannot be modified through the pointer

Syntax:

const data_type *pointer_name;

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5. Dangling Pointer

• A pointer that points to a memory location that has been freed or deleted

• Occurs when memory is released but the pointer still stores the old address

The following program will illustrate the pointer indirection operator and dereferencing

#include <stdio.h>
int main()
{
int num=5;
int *ptr=&num;
printf(“the value and address of num is %d %p\n”,num,&num);
printf(“the value and address of num using pointer ptr is %d %p\n”,*ptr,ptr);
*ptr=15;                     // indirection, which stores value in variable num using the pointer ptr.
printf(“the value of num after indirect initialization is %d\n”,num);
} 

the value and address of num is 5 0x7ffe6246e2fc

the value and address of num using pointer ptr is 5 0x7ffe6246e2fc

the value of num after indirect initialization is 15

The following program uses a call by reference mechanism to modify a variable value using pointers

#include <stdio.h>
void f(int *x) // x is a pointer to n
{
*x=*x+1; //accessing the actual content
}
int main()
{
int n=10;

f(&n); //calling the function with address/reference
printf("n in main =%d\n",n);
//note..... value of n changed in main
}

The following program use pointers to add two numbers using a  function add

int add(int *p1,int *p2)

{ 

          return(*p1+*p2);

 }

Write a function calculates the roots of a quadratic equation with coefficients a, b, and c (passed as pointer parameters).

#include <stdio.h>

int main()

{

    int a=10,b=12,sum;

    sum=add(&a,&b);

    printf("Sum=%d",sum);

}

#include <stdio.h>

#include <math.h>

void findRoots(float *a, float *b, float *c) {

    float discriminant, root1, root2, realPart, imagPart;

    discriminant = (*b) * (*b) - 4 * (*a) * (*c);

    if (*a == 0) {

        printf("Not a quadratic equation.\n");

        return;

    }

    printf("\nQuadratic equation: %.2fx^2 + %.2fx + %.2f = 0\n", *a, *b, *c);

    if (discriminant > 0) {

        root1 = (-(*b) + sqrt(discriminant)) / (2 * (*a));

        root2 = (-(*b) - sqrt(discriminant)) / (2 * (*a));

        printf("Real and distinct roots: %.2f and %.2f\n", root1, root2);

    }

    else if (discriminant == 0) {

        root1 = root2 = -(*b) / (2 * (*a));

        printf("Real and equal roots: %.2f and %.2f\n", root1, root2);

    }

    else {

        realPart = -(*b) / (2 * (*a));

        imagPart = sqrt(-discriminant) / (2 * (*a));

        printf("Complex roots: %.2f + %.2fi and %.2f - %.2fi\n",

               realPart, imagPart, realPart, imagPart);

    }

}

Write a C program to read an array of 5 integers.Use a pointer to find and print the largest element of the array.(university question)

#include <stdio.h>

int main() {

    int arr[5];

    int *ptr;

    int i, max;

    printf("Enter 5 integers:\n");

    for (i = 0; i < 5; i++) {

        scanf("%d", &arr[i]);

    }

    ptr = arr;          // Pointer points to the beginning of the array

    max = *ptr;         // Assume the first element is the largest

    for (i = 1; i < 5; i++) {

        if (*(ptr + i) > max) {

            max = *(ptr + i);

        }

    }

    printf("The largest element in the array is: %d\n", max);

    return 0;

}

Write a C program to concatenate two strings using pointers ( university question)

#include <stdio.h>

void concatenate(char *str1, char *str2) {

    // Move the pointer to the end of the first string

    while (*str1 != '\0') {

        str1++;

    }

    // Copy characters from str2 to the end of str1

    while (*str2 != '\0') {

        *str1 = *str2;

        str1++;

        str2++;

    }

    *str1 = '\0'; // Null-terminate the result

}

int main() {

    char str1[200], str2[100];

    printf("Enter the first string: ");

    fgets(str1, sizeof(str1), stdin);

    printf("Enter the second string: ");

    fgets(str2, sizeof(str2), stdin);

    // Remove newline characters if present

    for (int i = 0; str1[i] != '\0'; i++) {

        if (str1[i] == '\n') {

            str1[i] = '\0';

            break;

        }

    }

    for (int i = 0; str2[i] != '\0'; i++) {

        if (str2[i] == '\n') {

            str2[i] = '\0';

            break;

        }

    }

    // Concatenate using pointers

    concatenate(str1, str2);

    printf("Concatenated string: %s\n", str1);

    return 0;

}

Output

Enter the first string: abbc def

Enter the second string: hij kls

Concatenated string: abbc defhij kls