Determine whether a number contains specific prime factors

Question

Below is my code, and I can't get it to work the way it should.

I have to find prime numbers (this works fine). Then, if the prime numbers are 7 and 3
(63 = 7 * 3 * 3 or 7 = 7) the number is magical, and if it contains any others (98 = 7 * 7 * 2 or 42 = 7 * 3 * 2) it's not.

I'm kind of stuck here:

if (b != 7 && b != 3)

                        Console.WriteLine(k);
 else
                        Console.WriteLine(j);

I don't know how to fix it. Here is the whole code:

         string k="isnt magical";
        string j = "is magical";
        int a, b;
        Console.WriteLine("Vnesite svoje stevilo: ");
        a = Convert.ToInt32(Console.ReadLine());
        for (b = 2; a > 1; b++)/
            if (a % b == 0)
            {

                while (a % b == 0)
                {
                    a /= b;

                }


                if (b != 7 && b != 3)
                    Console.WriteLine(k);
                else
                    Console.WriteLine(j);
       }

Answer 1

You are printing "isnt magical" or "is magical" for every factor. Your code should look like this:

string k = "isnt magical";
string j = "is magical";
int a, b;
Console.WriteLine("Vnesite svoje stevilo: ");
a = Convert.ToInt32(Console.ReadLine());

var allMagical = true;
for(b = 2; a > 1; b++) if(a % b == 0)
{
    while(a % b == 0) a /= b;
    if(b != 7 && b != 3) allMagical = false;
}

Console.WriteLine(allMagical ? j : k);