Question

I don’t know why but it keeps saying that prime?(num) method is not defined…
I am trying to make it so that it finds the product of prime numbers between 1 and 20

class LCM200        
  def prime?(num)
    if num == 1 then 
        return false
    end
    range = Math.sqrt(num)
    for i in 2..range           
        if num%i == 0
            return false
        end     
        return true
    end
  end
#if __FILE__ == $0
  lcm = 1
  for j in 1..20 do
    if prime?(j)
      lcm *= j
    end
  end       
  puts lcm
#end
end

Answer 1

The issue is because you’re running the code inside a class. Inside a class, prime? gets defined as an instance method of the class, however when you’re running the for loop inside the class you’re calling prime? as though it were a class method (aka static method). To fix it, just remove the class declarations:

def prime?(num)
    if num == 1 then 
        return false
    end

    range = Math.sqrt(num)
    for i in 2..range

        if num%i == 0
            return false
        end
    end

    # Note: moved return true to here since otherwise it would return
    # true after a single iteration
    return true
end

lcm = 1

for j in 1..20 do
    if prime?(j)
        lcm *= j
    end
end

puts lcm

You can also define prime? in a more “Ruby-ish” way:

def prime? num
  if num == 1
    false
  else
    (2..Math.sqrt(num)).all? { |i| i % num != 0 }
  end
end

Documentation for Enumerable#all?