Help needed with IsPrime function.

[theflyingneko]

Hi!
So, I'm new to C++ and I have a final tomorrow. as i was practicing i encountered this problem: "Write a function, called IsPrime that receives one parameter n, and returns whether the number is prime or not."

So, I've tried it many times, and I get the same error while building it.
this is my code:

  #include <iostream>
using namespace std;
int IsPrime(int n)
{
	int c;
	for (int i = 2; i < n - 1; i++)
	{
		int c;
		if (n%i != 0)
			continue;
		else
			c = 0;
	}
	if (c == 0) 
		cout << "not prime.";
	else
		cout << "prime.";
	return c;
}
int main()
{
 int n;
	cout << "Please enter a number: ";
	cin >> n;
	IsPrime(n);

}

The problem is I get this error message that says: "Uninitialized local variable 'c' used" that is in line 14.
Please help...
Thank you.

[MistahMoose]

The Error message is straight forward.
On line 14, Variable 'c' is Uninitialized.
You declare a local variable in the for loop int c; and don't set it equal to anything.
So how are you supposed to compare c to something, when you do not know what c is?
Best practice is to Declare a variable, and set it to a value before you use it. Your Else statement may never be triggered and thus c may never equal 0. So it will have garbage in it

  #include <iostream>
using namespace std;
int IsPrime(int n)
{
	int c;//Declared int c here
	for (int i = 2; i < n - 1; i++)
	{
		int c;//Also declared int c here. Did not initialize either of them. Double declaration is bad
		if (n%i != 0)
			continue;
		else
			c = 0;//You set c equal to a value here
	}
	if (c == 0) 
		cout << "not prime.";
	else
		cout << "prime.";
	return c;//What is the purpose of returning c? Just set to void function since you print result in function
}
int main()
{
 int n;
	cout << "Please enter a number: ";
	cin >> n;
	IsPrime(n);

}

[AbstractionAnon]

You have two instances of the variable c. line 5 and line 8. The instance at line 8 hides the one at line 5. The instance at line 8 goes out of scope at line 13. At line 14, the instance at line 5 has never been initialized. Get rid of line 8.

A couple of suggestions on style:
1) Make IsPrime() a bool function.
2) Remove the printing from IsPrime().
3) Exit with false as soon as the first even division occurs. No need to check further.

#include <iostream>
using namespace std;

bool IsPrime(int n)
{   for (int i = 2; i < n - 1; i++)
	{   if (n%i == 0)
			return false;   //  No need to check further 
	}
	return true;    
}
int main()
{   int n;

	cout << "Please enter a number: ";
	cin >> n;
	if (IsPrime(n))
	    cout << "is prime.";
    else
        cout << "is not prime."; 	    
    return 0;
}