Question

I have a very basic question about Ruby loops.

This program as written returns the ith prime number +1 (ie the example should return 17). I know I could simply return cand-1, but I was wondering what the “Ruby way” of checking if the answer has been found at the bottom of the while loop and only incrementing if it hasn’t.

def ith_prime(i)
  pI = 0 # primes index
  divs = []
  cand = 2

  until pI == i do 
    if divs.find { |div| cand%div == 0 } == nil
        divs << cand
        pI += 1
    end
    cand += 1
  end
  cand
end

puts ith_prime(7)
> 18

Answer 1

I use loop instead of while or until most of the time. This way I can put the exit condition anywhere in the loop.

I would write it like that (if I understood the problem correctly):

def ith_prime(i)
  pI = 0 # primes index
  divs = []
  cand = 2

  loop do
    unless divs.find { |div| cand%div == 0 }
      divs << cand
      pI += 1
    end

    break if pI == i

    cand += 1
  end

  cand
end