Prime number program

You should be able to work this out, just break it down into smaller chunks

1) construct your for loop - don't forget you can increment by a factor of two
2) write print statement
3) See Divisibility rule[^] for the rules to see if a number has any simple factors
4) Check to see if the current number can be divided by any of the discovered primes so far

This should be enough to get you started

The Sieve of Eratosthenes[^] was always a favorite of mine :).
Another oldy but goodie is the Fermat Primality Test[^]. Since the Fermat Primality Test does have its flaws you might want to resort to this page, which lists other primality tests: Primality Test[^]

Best Regards,

—MRB

Google It

The first two links will give you sample code

Here is an implementation for the Sieve of Eratosthenes, There is actually a better Sieve algorithm, but this one (in contrary) is simple to understand.

bool *eratosthenes(int top)
{
    bool * sieve = new bool[top+1];

    // Initialize values

    sieve[0] = false; sieve[1] = false;
    for(int i=2;i<=top;i++)
        sieve[i] = true;

    /* it is sufficient to scan up to the sqrt, 
       because the inner loop would start at i*i. */
    for(int i=2;i <= sqrt(top); i++)
    {
        /*If i is not prime, the 2nd loop is redundant, because there was already p 
          that would have cancelled out both i and every other possible j. */
        if(!sieve[i])
            continue;

        /*we can start from i*i because i*2, ... i*(i-1) would already have 
          been assigned false in previous iterations by prime div of 2.,i-1. */
        for(int j=i*i;j<=top;j+=i)
            sieve[j] = false; // j isn't prime - i is a factor
    }
    return sieve;
}

int main()
{
bool isPrime=true;

//Loop through all the numbers 3-100
for(int primeCandidate=3; primeCandidate<=100; primeCandidate++)
{
//Reset isPrime to true
isPrime = true;

//Check the factors
for(int factor=2; factor<primecandidate-1;>{
//If a factor is found, the number is not prime. No reason to check
//the rest of the numbers, so break out of this 'for' loop.
if(primeCandidate%factor == 0)
{
isPrime = false;
break;
}
}

//If the number is prime, print it.
if(isPrime == true)
{
cout<< primeCandidate << " is prime." << endl;
}
}

return 0;
}