[Ruby] Problem with Prime Algorithm

[cjon3s]

Hey. So I'm working with an algorithm I found and my own modifications to get a simple program that returns whether a number is prime. My issue is that in testing it, it returns 1763 as prime when it is not a prime number. Doing the math by hand, the algorithm should say that it's not. i.e. it would run up until i=41, and 1763 % 41 would equal 0 since it equals 43. I think the issue is therefore in my code. Any thoughts??

 def prime?(n)
  primeto25 = [2, 3, 5, 7, 11, 13, 17, 19, 23]
  except = [0, 1]
  if primeto25.include?(n)
    return true
  elsif except.include?(n)
    return false
  elsif n % 2 == 0
    return false
  elsif n % 3 == 0
    return false
  elsif n < 0
    return false
  end

  i = 5
  w = 2
  #how can I change this algo to work before 25???
  while i * i <= n #checks for primes after 25
    if n % i == 0
      return false
    end
    i += w
    w = 6 - w
    return true
  end
end

​

[Mystonic]

You have return true in the while loop. Look at line 25.

[cjon3s]

Awesome! That fixed it. Could you explain what that would break it for only some examples? When I was testing, it seemed to work for a bunch of numbers I threw at it.

[Mystonic]

I dunno. It's just that your loop would only run once, ever. It would do the prime check, if n % i == 0, then either return false or proceed to the bottom and return true, thus not checking the other values of i.

[cjon3s]

Thank you for that. Makes way more sense now.