Programming Forums > Application Development > C++

cairo · Jun 2nd, 2006, 1:39 PM

bool isPrime(int num)
{
	bool isPrimeNum;

	int sqrtNum;
	int divisor = 3;

	if (num == 2)		   	
		isPrimeNum = true;
	else
		if (num % 2==0)
			isPrimeNum=false;
		else
		{
			isPrimeNum = true;

			sqrtNum = static_cast<int>(pow(static_cast<double>(num), 0.5));
			while (divisor <= num )
			{
				if (num % divisor == 0 )
				{
					isPrimeNum = false;
					cout << " not prime number"<<endl;
					break;
				}
				else
					divisor = divisor + 2;
			}
		}

		return isPrimeNum;
}

im going to crack my head because of this program, i failed to solve the problem, i cant print out the " is not prime " message.
any idea, guys ?

NSchnarr · Jun 2nd, 2006, 2:17 PM

Is there any reason for the sqrtNum variable, i cant see it doing anything.

Twilight · Jun 2nd, 2006, 2:25 PM

When I ran your code, the cout worked fine.

And your code as of now works backwards. It just thinks primes are not primes, and vise versa.

Harakim · Jun 2nd, 2006, 2:43 PM

A few things.

in your loop, while ( divisor <= num ) means that if you chose, say 37 and your divisor got to 37, the loop would execute and state that it is not a prime when it is. You need to change `num` to `sqrtNum` or `<=` to `<`

If you are testing with even numbers, the cout will never get called:

(Toggle Plain Text)

if (num % 2==0)
	isPrimeNum=false;

If you want to make it much more efficient for large numbers, you could compile a list of primes up to 2^16 (by cut and paste/download from the internet) and check against that list up to the square root of the number. It would use more memory, but hardly touch the cpu. If it's not important, this would probably be a waste of time.

EDIT: Also try putting your num % xxx in parenthesis(sp?).

Jun 2nd, 2006, 1:39 PM	#1
cairo Newbie Join Date: Dec 2005 Posts: 18 Rep Power: 0	prime number, problem (Toggle Plain Text) bool isPrime(int num) { bool isPrimeNum; int sqrtNum; int divisor = 3; if (num == 2) isPrimeNum = true; else if (num % 2==0) isPrimeNum=false; else { isPrimeNum = true; sqrtNum = static_cast<int>(pow(static_cast<double>(num), 0.5)); while (divisor <= num ) { if (num % divisor == 0 ) { isPrimeNum = false; cout << " not prime number"<<endl; break; } else divisor = divisor + 2; } } return isPrimeNum; } bool isPrime(int num) { bool isPrimeNum; int sqrtNum; int divisor = 3; if (num == 2) isPrimeNum = true; else if (num % 2==0) isPrimeNum=false; else { isPrimeNum = true; sqrtNum = static_cast<int>(pow(static_cast<double>(num), 0.5)); while (divisor <= num ) { if (num % divisor == 0 ) { isPrimeNum = false; cout << " not prime number"<<endl; break; } else divisor = divisor + 2; } } return isPrimeNum; } im going to crack my head because of this program, i failed to solve the problem, i cant print out the " is not prime " message. any idea, guys ?

Jun 2nd, 2006, 2:25 PM	#3
Twilight Programmer Join Date: Apr 2006 Location: Calgary, Alberta Posts: 67 Rep Power: 3	When I ran your code, the cout worked fine. And your code as of now works backwards. It just thinks primes are not primes, and vise versa. Last edited by Twilight; Jun 2nd, 2006 at 2:47 PM.

Jun 2nd, 2006, 2:43 PM	#4
Harakim Hobbyist Programmer Join Date: May 2006 Location: West Jordan, Utah, United States Posts: 176 Rep Power: 3	A few things. in your loop, while ( divisor <= num ) means that if you chose, say 37 and your divisor got to 37, the loop would execute and state that it is not a prime when it is. You need to change `num` to `sqrtNum` or `<=` to `<` If you are testing with even numbers, the cout will never get called: (Toggle Plain Text) if (num % 2==0) isPrimeNum=false; if (num % 2==0) isPrimeNum=false; If you want to make it much more efficient for large numbers, you could compile a list of primes up to 2^16 (by cut and paste/download from the internet) and check against that list up to the square root of the number. It would use more memory, but hardly touch the cpu. If it's not important, this would probably be a waste of time. EDIT: Also try putting your num % xxx in parenthesis(sp?).

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Jun 2nd, 2006, 2:17 PM	#2
NSchnarr Newbie Join Date: May 2006 Posts: 28 Rep Power: 0	Is there any reason for the sqrtNum variable, i cant see it doing anything.

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Programming Forums > Application Development > C++

prime number, problem