Thread: Prime number program

Been a long time since I posted a problem aye! Still remember ol Prelude helping me with that termination prevention thingy.
So, to the point. I'm making a prime numbers program and in two cases I do the following -:
2. Enter a limit upto which primes will be displayed and summed.
3. Enter 'n' for which n primes will be displayed and summed.
I do get the display but not the sum. Can't seem to figure out whats wrong.
Here's case 2-:

Code:

 case 2:
                       cout << "\nEnter any limit";
                       cin >> l; //The limit
                       for(num = 2; num <= l; num++) //num begins at 2 and keeps incrementing till l
                       {
                               flag = 1; //flag variable
                               for(i = 2; i <= num/2; i++)
                               {
                                     if (num % i == 0)
                                     flag = 0;
                                     break;
                               }
                               if(flag == 1)
                               cout << num << "\t";
                               sum = sum + num;
                       }
                             
                               cout << "The sum is -> " << sum;
                               getch();
                               break;

And case 3-:

Code:

case 3:
                       cout << "\nEnter the number of terms - ";
                       cin >> n;
                       ctr = 0; //counter variable
                       for(num = 2; ctr < n; num++)
                       {
                               flag = 1;
                               for(i = 2; i <= num/2; i++)
                               {
                                     if(num % i == 0)
                                     flag = 0;
                                     break;
                               }
                               if(flag == 1)
                               cout << num << "\t";
                               sum += num;
                               ctr++;
                       }
                               cout << "The sum is -> " << sum;
                               getch();
                               break;

And the variables have been declared before hand.

To do case three correctly you must compute n primes beforehand. You might first factor out the code that does work in case two as a new function and then use it to build a table of the first 1000 primes.

After you calculate the primes, use the table and the function together to complete cases two and three. The difference is in case two you will sum all primes up to n. The difference in case three is that you will be computing up to n terms of an arithmetic series, and to do that you need a bunch of primes ready beforehand.

For example the sum of the first five terms is 18 = 1 + 2 + 3 + 5 + 7.

here is a little modified case 2 program, tested .....

Code:

#include<iostream.h>
#include<conio.h>
void main() {
 cout << "\nEnter any limit";
 int l;
 cin >> l; //The limit
 int num, sum=0, flag, i;
 for(num = 2; num <= l; num++) //num begins at 2 and keeps incrementing till l
	  {
				 flag = 1; //flag variable
			 for(i = 2; i <= num/2; i++)
				 {
						 if (num &#37; i == 0)  {
				 flag = 0;
						 break;
						 }
				 }
		 if(flag == 1) {
			 cout << num << "\t";
			 sum = sum + num;
				 }
	 }
		 cout << "The sum is -> " << sum;
		 getch();
}

SVXX, gvkalra is a chronic includer of the pre-standard <iostream.h> header and a chronic void main()er (the global main function should return an int), so think twice before copying his/her code. Oh, and your indentation is much better than gvkalra's too.

Instead I suggest that you read through citizen's post and make use of the suggestions there.