Thread: Prime Checking Algorithm

I'm still new to C. I wrote some code, but I'm sure I did something wrong because it's not giving the correct output.

(compiling and running on linux)

Code:

void main()
{
	unsigned long startingNumber; // THE Number. Is it prime?

	printf("Enter Positive Number (0 to 4,294,967,295) to be Tested : ");
	scanf(startingNumber);
	printf("\nTesting ", startingNumber, "...\n");

	unsigned long testNumber = (startingNumber / 2);
	// A number can't be evenly divided by a number bigger than half the first number.

	unsigned long answer = startingNumber % testNumber; // Get the initial remainder.
	
	while (answer != 0 && testNumber > 1) // Decrement while checking for a modulus of 0.
	{
		answer = startingNumber % testNumber;
		testNumber = testNumber - 1;
	}

	if ((answer == 0) && ((startingNumber / 2) > 1) || (startingNumber == 2))
	{
		// Check for prime, with fixes for 2, and testNumbers 1 or smaller.
		printf(startingNumber, " is NOT a PRIME Number\n");
	}
	else
		printf(startingNumber, " is a PRIME Number\n");
	return 0;
};

Output:

Code:

fuzzypig@fuzzypig-ubuntu:~/Documents/source/primechk/c$ ./a.out
Enter Positive Number (0 to 4,294,967,295) to be Tested : 265952  

Testing Hfῼﾛ�
fuzzypig@fuzzypig-ubuntu:~/Documents/source/primechk/c$

Using GDB, the program exits with code 011.
Can someone help me fix my code?

I tried to follow that, but it's still not working.

New code:

Code:

int main(void)
{
	unsigned long startingNumber; // THE Number. Is it prime?
	unsigned long testNumber;
	unsigned long answer;

	printf("Enter Positive Number (0 to 4,294,967,295) to be Tested : \n");
	scanf("%i", &startingNumber);
	printf("Testing \n", startingNumber);

	testNumber = (startingNumber / 2);
	// A number can't be evenly divided by a number bigger than half the first number.

	answer = startingNumber % testNumber; // Get the initial remainder.
	
	while (answer != 0 && testNumber > 1) // Decrement while checking for a modulus of 0.
	{
		answer = startingNumber % testNumber;
		testNumber = testNumber - 1;
	};

	if ((answer == 0) && ((startingNumber / 2) > 1) || (startingNumber == 2))
	{
		// Check for prime, with fixes for 2, and testNumbers 1 or smaller.
		printf(startingNumber, " is NOT a PRIME Number\n");
	};
	else
		printf(startingNumber, " is a PRIME Number\n");
	return 0;
};

I still get several compiler warnings:

Code:

primechk.c: In function ‘main’:
primechk.c:9: warning: incompatible implicit declaration of built-in function ‘printf’
primechk.c:10: warning: incompatible implicit declaration of built-in function ‘scanf’
primechk.c:10: warning: passing argument 1 of ‘scanf’ makes pointer from integer without a cast
primechk.c:27: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast
primechk.c:30: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast
primechk.c:31: warning: ‘return’ with a value, in function returning void
primechk.c:6: warning: return type of ‘main’ is not ‘int’

It's cool that fuzzypig also uses ubuntu !
It's rather ideal for gcc.

I've written prime programs before but I really better not post them.

Originally Posted by honorable_sir

It's cool that fuzzypig also uses ubuntu !
It's rather ideal for gcc.

Strange thing to say. I'd say ANY Linux, or for that matter any platform where gcc works, is "ideal for gcc"

You can stop checking whether the number is prime or not after testing up to, and including, the square root of the number. If it's still prime at that point, then it will stay prime all the way.

Big speed up if you're trying to find lots of prime numbers.