Question

Below is some code from Codeacademy.com. There is something I don’t understand: Why does the value of is_prime not change with every iteration of the loop? For example, testing the number 8, it may be that the loop results in is_prime being false for values 2 and 3, but then for 4 it becomes false (because you can divide by 4). However, for 5,6 and 7 its true again. So why at the end of the loop does the value stay false even after 5,6 and 7 would make it be true again?

def prime(n)
  puts "That's not an integer." unless n.is_a? Integer
  is_prime = true
  for i in 2..n-1
    if n % i == 0
      is_prime = false
    end
  end
  if is_prime
    puts "#{n} is prime!"
  else
    puts "#{n} is not prime."
  end
end

prime(2)
prime(9)
prime(11)
prime(51)
prime(97)

Answer 1

The behavior you’re describing is what would happen if it was written like this:

for i in 2..n-1
  if n % i == 0
    is_prime = false
  else
    is_prime = true
  end
end

But in your code there is no else. If the number is not divisible, nothing happens so the variable stays like it was.