prime numbers

quasimof · #1 01-Nov-2004, 14:53

i wrote this program to find prime numbers, but whenever i run it, i get an error.
This is the code [c++]:

CPP / C++ / C Code:

int main(void)
{

int num=0;
int i=0;

cout<<"num";
cin>>num;

while(i<num)
{
  num=num%i;
  i++;
}
  if(num==0)
      cout<<"not prime";

  else 
      cout<<"prime";

return 0;
}

My real task is to get the user to input a number greater than 2, and then print out what prime numbers added together would make up the number the user inputed. For example:

4 = 2 + 2
6 = 3 + 3
8 = 3 + 5
10 = 3 + 7 etc.

i need to create a function: void Goldbach(int n, int& p1, int & p2)

and the main program would be:

CPP / C++ / C Code:

void main()
{
    int num, prime1, prime2;
    cout<<"Enter an even positive integer greater than 2:";
    cin>>num;
    Goldbach(num,prime1,prime2);
    cout<<num<<" = "<<prime1<<" + "<<prime2<<endl;

}

//my first step is to determine what 2 prime numbers would add up to get the //number the user inputs.

dsmith · #2 01-Nov-2004, 20:35

Hi quasimof. Your program is giving you an error, because the first time through your loop i is 0. You are dividing num by i and any number divided by 0 is... undefined.

Anyway, I don't think your logic is sound either. If you search these forums you should be able to find several threads that have good prime number finding algorithms such as this one here.

Once you get your prime number tester working, your other function should fall right into place

Good luck!

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