Prime Number Calculator

[Vasiliy Slobodov]

Hi! I'm having a bit of an issue with my prime number counter/calculator.

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

using namespace std;

bool isprime (int a)
{

int c;
bool first;

 for (int b = 2; b < a; b++ ) {
	c = (a%b);
	if (c == 0) {
		first = 0; } 
}	


if (first == 1)  {
	cout << "The number " << a << " is prime." << endl; }
else
	cout << "The number " << a << " is not prime." << endl; 

return first; 

}

int main ()
{

int x, y, z;

start:
cout << "Would you like to test a range of numbers for being prime, or just one number." << endl << setw (4) << "(1) Range of numbers." << endl << setw(4) << "(2) Just one number." << endl << endl << "Make your selection: ";
cin >> x;

switch (x) {

case 1: 
	cout << "Please enter the number up to which you'd like to get the output:"; 
	cin >> y;
		for (int g = 1; g <= y; g++) {
			isprime (g); }	
	break;
case 2: 
	cout << "Enter the number you'd like to test: " ;
	cin >> y;

isprime (y);
	break;

default: cout << "You have made an invalid selection. Please press enter to try again." << endl << endl;
	 cin.get ();
	 cin.get ();
	 goto start;
}

cin.get ();
cin.get ();

return 0;
}

So, it works perfectly fine when I select option 2. It reads the number, and returns a correct value. I tested it for a whole bunch of numbers. Buut, here's the problem. When I do the range option, it reads the first three numbers (1, 2, 3) as primes, which is correct, then 4 as a non-prime, and then 5 as a non-prime as well.

Wait, what?

Output looks like this:

Make your selection: 1
Please enter the number up to which you'd like to get the output:10
The number 1 is prime.
The number 2 is prime.
The number 3 is prime.
The number 4 is not prime.
The number 5 is not prime.
The number 6 is not prime.
The number 7 is not prime.
The number 8 is not prime.
The number 9 is not prime.
The number 10 is not prime.

What I think is happening is that the boolean prime is not clearing every time the function finishes executing. So, is there a way to 'clear' the function after it's used? Would isprime(void) work in the main ()?

EDIT: Well, I dunno if it counts as solved, but I did manage to do a workaround. I simply placed a first = 1; right before the return first statement.

[coder777]

Your isprime() has 2 problems:

1. The variable 'first' on line 12 is uninitialized. It might be true or false. So if the loop detect it is actually a prime (first is not set in this case) you get randomly 0 or 1. Solution on line 12: bool first = true;

2. The loop on line 14 doesn't stop when it detect that it's not prime. Add a break; after first = 0; on line 17