Problem with nested loop in prime sieve program

[physicstom1]

Hello all,

I'm new to the forums and C++ (having previously programmed in fortran). I've been trying to write a program to print out the prime numbers up to a value N. When debugging the program I get the following warning:

primes.cc:25:8: warning: expression result unused [-Wunused-value]
for (f; f != 0; f--) {
^
(Note: I am using unix)

When running the program, I am prompted to input a value of N as expected, but when I press return, the program ends. I think the problem may lie in the break statement:

if (modulo == 0) break;

Despite reading that 'break' will end only the nested loop and not both, I see it as the only thing that might be going wrong. When I replace this with 'continue', the program prints out all integers from N to 0, so if I input N=20, the output is 20, 19, 18, ... 3, 2,

The code follows, any help/advice for a total C++ novice would be greatly appreciated.

Thanks in advance,

Tom

  	int N, f;
	float modulo;

	cout << "Welcome to the prime number sieve\n";
	cout << "Enter the number up to which the prime numbers will be printed: ";
	cin >> N;

	for (int j = N; j != 1; j--) {              
		f = j;

		for (f; f != 0; f--) {
		   modulo = j%f;
		  	if (f == 1) {
		  		cout << j << ", ";
		  	}	
		  	if (modulo == 0) break;
			}						                  
	
	}

  return 0;

EDIT: I should note that this is just the relevant portion of the code, I have omitted the irrelevant parts