Pointer, Structure and Union

 

1. Simple Pointer Example

This program demonstrates the basic usage of a pointer.

#include <stdio.h>

int main() {
    int x = 10;
    int *p;   // Declaration of pointer

    p = &x;   // Assign the address of x to pointer p

    printf("Value of x: %d\n", x);
    printf("Address of x: %p\n", &x);
    printf("Value of p (Address stored in p): %p\n", p);
    printf("Value at address p (Value of *p): %d\n", *p);

    return 0;
}

2. Pointer to Array
This program shows how a pointer can be used to access elements of an array.

#include <stdio.h>

int main() {
    int arr[5] = {10, 20, 30, 40, 50};
    int *p;

    p = arr;  // p now points to the first element of the array

    for(int i = 0; i < 5; i++) {
        printf("Element %d: %d\n", i+1, *(p + i));
    }

    return 0;
}
3. Pointer to Pointer
This example demonstrates a pointer to a pointer.

#include <stdio.h>

int main() {
    int x = 100;
    int *p;
    int **pp;

    p = &x;   // Pointer p stores the address of x
    pp = &p;  // Pointer to pointer pp stores the address of p

    printf("Value of x: %d\n", x);
    printf("Value at p (Value of *p): %d\n", *p);
    printf("Value at pp (Value of **pp): %d\n", **pp);

    return 0;
}
4. Pointer Arithmetic
This program demonstrates pointer arithmetic.

#include <stdio.h>

int main() {
    int arr[3] = {1, 2, 3};
    int *p;

    p = arr;  // p now points to the first element of the array

    printf("Address of p: %p, Value at p: %d\n", p, *p);
    p++;
    printf("Address after p++: %p, Value at new p: %d\n", p, *p);
    p--;
    printf("Address after p--: %p, Value at new p: %d\n", p, *p);

    return 0;
}
5. Function Pointers
This example shows how to use pointers to functions.

#include <stdio.h>

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

int main() {
    void (*funcPtr)();  // Declaration of a function pointer

    funcPtr = displayMessage;  // Assign address of the function

    // Call the function using the pointer
    (*funcPtr)();

    return 0;
}



Structure in C
A structure in C is a user-defined data type that allows grouping variables of 
different types under a single name.
Example Code: Structure

#include <stdio.h>

// Defining a structure
struct Student {
    int rollNumber;
    char name[50];
    float marks;
};

int main() {
    // Declaring a structure variable
    struct Student student1;

    // Assigning values to the structure members
    student1.rollNumber = 1;
    strcpy(student1.name, "John Doe");
    student1.marks = 85.5;

    // Accessing structure members
    printf("Student Roll Number: %d\n", student1.rollNumber);
    printf("Student Name: %s\n", student1.name);
    printf("Student Marks: %.2f\n", student1.marks);

    return 0;
}
Union in C
A union in C is similar to a structure, but it allows storing different 
data types in the same memory location. This means only one member can contain 
a value at any given time.
Example Code: Union

#include <stdio.h>

// Defining a union
union Data {
    int i;
    float f;
    char str[20];
};

int main() {
    // Declaring a union variable
    union Data data;

    // Assigning and accessing integer value
    data.i = 10;
    printf("data.i: %d\n", data.i);

    // Assigning and accessing float value
    data.f = 220.5;
    printf("data.f: %.2f\n", data.f);

    // Assigning and accessing string value
    strcpy(data.str, "C Programming");
    printf("data.str: %s\n", data.str);

    // Notice that the value of `data.i` and `data.f` has been overwritten
    printf("data.i after str assignment: %d\n", data.i);
    printf("data.f after str assignment: %.2f\n", data.f);

    return 0;
}
Key Differences Between Structure and Union:
Memory Allocation: A structure allocates memory for all its members, 
whereas a union allocates memory equal to the size of its largest member.
Data Storage: All members of a structure can store data simultaneously, 
but only one member of a union can store data at a time.