Question

I have this piece of code which checks whether a given number is prime:

If x Mod 2 = 0 Then
    Return False
End If
For i = 3 To x / 2 + 1 Step 2
    If x Mod i = 0 Then
        Return False
    End If
Next
Return True

I only use it for numbers 1E7 <= x <= 2E7. However, it is extremely slow – I can hardly check 300 numbers a second, so checking all x‘s would take more than 23 days…

Could someone give some improvements tips or say what I might be doing redundantly this way?

Answer 1

You should definitely change your algorithm! You can try Sieve of Eratosthenes or a more advanced Fermat primality test. Beware that your code will be more complicated, as you would need to implement modular arithmetics. Look here for the list of some even more mathematically advanced methods.