2prime.cpp

some useful programs and simple yet

#include<stdio.h>
#include<conio.h>
#include<math.h>
/*
   Written By: Morteza Zafari 2001
   This program calculates all possible combination of two
   prime numbers which produce a given even number.
*/

int IsPrime(int);  //returns 1 if prime.  returns 0 if not prime.

void main()
	{
	int i,r,num;
	clrscr();
	printf ("_____________________________________________________________\n");
	printf (" This program will calculate all possible combination of two\n prime numbers which produce a given even number. \n");
	printf ("_____________________________________________________________\n");
	printf ("\nPlease input an even number: ");
	scanf  ("%d",&num);    // Getting a number from the user.
	if (num%2 != 0)        // Checking if the given number is even or not.
	   {
	   printf("\n%d is not an even number",num);
	   getch();
	   return;  //Exit the program.
	   }
	//---------- Start -----------//

	/*
	    This program uses just one 'For' statement
	    as we know 'A+(B-A)=B' so if 'A' AND '(B-A)'
	    are prime numbers, they are the answer.
	*/
	for (i=2;i<=(num/2);i++)
	   {
	   if ( IsPrime(i) && IsPrime(num-i) )
	      {                // If BOTH of them are prime, Show them.
	      printf ("\n %4d + %4d = %d ",i,(num-i),num);
	      }
	   }
	printf("\n\n Thanks for using this program.\n Morteza Zafari\n 2001");
	getch();
	}
	//---------- End -----------//

IsPrime(int num)
   {
   int i;
   if (num==1) return(0);    //If it's 1, it's not prime.
   if (num==2) return(1);    //If it's 2, it's prime.
   if (num%2==0) return(0);  //If it is even then sure it's not prime.
   else
      {
      for (i=2;i<=sqrt(num);i++)
	{
	 if ( (num%i) == 0 ) return(0);
	}
	return(1);
      }
   }