determining prime numbers

Turkeycatsmom (2)
Hi,

I'm new with C++ and need help with prime numbers. I need to write a program that determines whether a number is a prime number or not. Here's what my code currently looks like:

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#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;


int IsPrime(int num)
{
	for(int i = 2; i <= sqrt((double)num); i++)
	{
		int ans;

		if((num % i) == 0 && (i!=1 && i!=num))
			ans = 0;
		else
			ans = 1;
		return ans;
	}
}

int main ()
{
int Number;
char ContinueLoop = 'Y';

while (ContinueLoop == 'Y')
{
		cout <<endl<<endl 
		<<"Please enter a number:  ";
		cin >> Number;
		
		{
		int ans = IsPrime(Number);
			if (ans == 0)
				cout<<"The number is not a prime number";
			else
				cout<<"The number is a prime number";
		}
	
	cout<<"\nDo you want to continue? (Y)  ";
	cin>>ContinueLoop;
	cout<<endl;
	ContinueLoop = toupper(ContinueLoop);
}//end of ContinueLoop
	
	cout<<"\nGood-bye!";

return 0;
}

When I test 15 and 21, my code claims they are prime number (ugh!). Any suggestions for how to determine a prime or if I did something wrong with my code would be appreciated. Thank you
Last edited on
closed account (z05DSL3A)
At the top of this page,do a search for 'Prime' there has been a lot of questions answered about it.
msram (22)
Please use this code. 
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int IsPrime(int num)
{
        int ans=1;

	for(int i = 2; i <= num/2; i++)
	{
		if(num % i == 0)
               {
			ans = 0;
                        break;
               }	
	}

        return ans;
}


PS: This is not the fastest program. For a discussion on the time/space complexity of PRIMES, please see the thread: "Prime number program" by doom at http://www.cplusplus.com/forum/general/1125/
closed account (z05DSL3A)
@msram,
Being pedantic, your code would say that any negative number, zero, and one is prime.
msram (22)
@Grey Wolf: yes... need to check for those basic conditions. Thank you for the comment.

@Turkeycatsmom: Please note Grey Wolf's comment.
Last edited on
Turkeycatsmom (2)
msram and Grey Wolf: msram's code worked for me. I appreciate all the help you provided. Thank you again.
msram (22)
Turkeycatmom: In addition to being able to use this code, please understand why your code does not work. That would help you write your code correctly on your own.
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