Check whether a year is leap year or not.

#include <stdio.h>

#include <conio.h>

void main()

{

  int year;

  clrscr();

  printf("Enter a year to check if it is a leap year\n");

  scanf("%d", &year);

  if ( year%400 == 0)

    printf("%d is a leap year.\n", year);

  else if ( year%100 == 0)

    printf("%d is not a leap year.\n", year);

  else if ( year%4 == 0 )

    printf("%d is a leap year.\n", year);

  else

    printf("%d is not a leap year.\n", year); 

  getch();

}

Output of program:

Find out the factorial of positive number

For any positive number n, its factorial is given by:

factorial = 1*2*3*4....n

If a number is negative, factorial does not exist and factorial of 0 is 1.

This program takes an integer from a user. If user enters negative integer, this program will display error message and if user enters non-negative integer, this program will display the factorial of that number.

Source Code to Find Factorial of a Number

/* C program to display factorial of an integer if user enters non-negative integer. */

#include <stdio.h>

#include <conio.h>

void main()

{

    int n, count;

    clrscr();

    unsigned long long int factorial=1;        

    printf("Enter an integer: ");

    scanf("%d",&n);

    if ( n< 0)

        printf("Error!!! Factorial of negative number doesn't exist.");

    else

    {

       for(count=1;count<=n;++count)    /* for loop terminates if count>n */

       {

          factorial*=count;              /* factorial=factorial*count */

       }

    printf("Factorial = %lu",factorial);

    }

    getch();

}

Output 1

Enter an integer: -5

Error!!! Factorial of negative number doesn't exist.

Output 2

Enter an integer: 10

Factorial = 3628800

Here the type of factorial variable is declared as: unsigned long long. It is because, the factorial is always positive, so unsigned keyword is used and the factorial of a number can be pretty large. For example: the factorial of 10 is 3628800 thus, long long keyword is used. 

To print the reverse a number (digit).

This program reverse the number entered by the user and then prints the reversed number on the screen. For example if user enter 123 as input then 321 is printed as output. In our program we use modulus(%) operator to obtain the digits of a number. To invert number look at it and write it from opposite direction or the output of code is a number obtained by writing original number from right to left. To reverse or invert large numbers use long data type or long long data type if your compiler supports it, if you still have large numbers then use strings or other data structure.

C programming code

#include <stdio.h>

#include <conio.h>

void main()

{

   int n, reverse = 0;

   clrscr();

   printf("Enter a number to reverse\n");

   scanf("%d", &n);

   while (n != 0)

   {

      reverse = reverse * 10;

      reverse = reverse + n%10;

      n       = n/10;

   }

   printf("Reverse of entered number is = %d\n", reverse);

   getch();

}