struct structure_name {
    data_type member1;
    data_type member2;
    ...
};

struct Student {
    int roll;
    char name[20];
    float marks;
};

struct Inner {
    data_type member1;
};

struct Outer {
    data_type member;
    struct Inner obj;   /* nested structure member */
};

struct Date {
    int day;
    int month;
    int year;
};

struct Student {
    int roll;
    char name[20];
    struct Date dob;    /* nested structure */
};

#include <stdio.h>
#include <conio.h>

/* Function to generate the nth term of Fibonacci series */
int fibonacci(int n)
{
    int first = 0, second = 1, next, i;

    if (n == 1)
        return 0;
    if (n == 2)
        return 1;

    for (i = 3; i <= n; i++)
    {
        next = first + second;
        first = second;
        second = next;
    }
    return next;
}

/* Function to check whether a number is prime or not */
/* Returns 1 if prime, 0 if not prime */
int isPrime(int num)
{
    int i;

    if (num <= 1)
        return 0;   /* 0 and 1 are not prime */

    for (i = 2; i <= num / 2; i++)
    {
        if (num % i == 0)
            return 0;   /* divisible, so not prime */
    }
    return 1;   /* prime */
}

/* Main function */
void main()
{
    int n, term, result;

    clrscr();

    printf("Enter the value of n: ");
    scanf("%d", &n);

    /* Generate nth Fibonacci term */
    term = fibonacci(n);
    printf("\nThe %dth term of Fibonacci series is: %d\n", n, term);

    /* Check if the term is prime */
    result = isPrime(term);

    if (result == 1)
        printf("%d is a PRIME number.\n", term);
    else
        printf("%d is NOT a prime number.\n", term);

    getch();
}

Enter the value of n: 7

The 7th term of Fibonacci series is: 8
8 is NOT a prime number.

Enter the value of n: 5

The 5th term of Fibonacci series is: 3
3 is a PRIME number.

int arr[5] = {10, 20, 30, 40, 50};
int *ptr = arr;   // ptr points to the first element of arr

#include<stdio.h>
#include<conio.h>

/* Function prototypes */
void callByValue(int x);
void callByReference(int *x);

void main()
{
    int a = 15;
    int b = 15;

    clrscr();

    /* Demonstrating Call by Value */
    printf("--- Call by Value ---\n");
    printf("Before calling function, a = %d\n", a);
    callByValue(a);
    printf("After calling function, a = %d\n\n", a);

    /* Demonstrating Call by Reference */
    printf("--- Call by Reference ---\n");
    printf("Before calling function, b = %d\n", b);
    callByReference(&b);
    printf("After calling function, b = %d\n", b);

    getch();
}

/* Call by Value: works on a copy */
void callByValue(int x)
{
    x = x + 5;
    printf("Inside callByValue function, x = %d\n", x);
}

/* Call by Reference: works on original via pointer */
void callByReference(int *x)
{
    *x = *x + 5;
    printf("Inside callByReference function, *x = %d\n", *x);
}

--- Call by Value ---
Before calling function, a = 15
Inside callByValue function, x = 20
After calling function, a = 15

--- Call by Reference ---
Before calling function, b = 15
Inside callByReference function, *x = 20
After calling function, b = 20

source.c --> Compiler --> source.obj --> Linker --> source.exe --> Output

#include <stdio.h>
#include <conio.h>

/* Define the structure Book */
struct Book {
    char Book_Name[50];
    float Price;
    char Author[50];
};

void main() {
    int i;

    /* Declare an array of 10 Book structures */
    struct Book books[10];

    clrscr();

    /* Input 10 book records */
    printf("Enter details of 10 books:\n");
    for (i = 0; i < 10; i++) {
        printf("\n--- Book %d ---\n", i + 1);

        printf("Enter Book Name  : ");
        scanf(" %[^\n]", books[i].Book_Name);

        printf("Enter Author Name: ");
        scanf(" %[^\n]", books[i].Author);

        printf("Enter Price      : ");
        scanf("%f", &books[i].Price);
    }

    /* Print authors whose book price is greater than 1000 */
    printf("\n\nAuthors with Book Price greater than 1000:\n");
    printf("--------------------------------------------\n");

    for (i = 0; i < 10; i++) {
        if (books[i].Price > 1000) {
            printf("Author: %s  |  Book: %s  |  Price: %.2f\n",
                   books[i].Author,
                   books[i].Book_Name,
                   books[i].Price);
        }
    }

    getch();
}

Enter details of 10 books:

--- Book 1 ---
Enter Book Name  : Let Us C
Enter Author Name: Yashavant Kanetkar
Enter Price      : 850

--- Book 2 ---
Enter Book Name  : The C Programming Language
Enter Author Name: Dennis Ritchie
Enter Price      : 1200

--- Book 3 ---
Enter Book Name  : Data Structures
Enter Author Name: Seymour Lipschutz
Enter Price      : 1500
...

Authors with Book Price greater than 1000:
--------------------------------------------
Author: Dennis Ritchie   |  Book: The C Programming Language  |  Price: 1200.00
Author: Seymour Lipschutz|  Book: Data Structures             |  Price: 1500.00

pointer_variable = (datatype *) malloc(specified_size);

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *ptr;
    int n, i;

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

    /* Dynamically allocate memory for n integers */
    ptr = (int *) malloc(n * sizeof(int));

    /* Check if memory was allocated successfully */
    if (ptr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }

    /* Input values */
    printf("Enter %d integers:\n", n);
    for (i = 0; i < n; i++) {
        scanf("%d", &ptr[i]);
    }

    /* Display values */
    printf("You entered:\n");
    for (i = 0; i < n; i++) {
        printf("%d ", ptr[i]);
    }
    printf("\n");

    /* Release the allocated memory */
    free(ptr);

    return 0;
}

Enter number of elements: 4
Enter 4 integers:
10 20 30 40
You entered:
10 20 30 40