Why this % Error in Ruby ? Beginner Learning-CodePudding

I just began with Ruby yesterday. I saw some basics online and started to do some beginner coding challenges and one asks for finding prime numbers. I try not using the prime feature already in Ruby. I think I almost have it, but I don't understand why the error shows itself.

"18:in ': undefined method %' for nil:NilClass (NoMethodError)"

I tried to tweak around it for a few hours now, and can't seem to find solutions to this online... I use Ruby 3.0.0, here's the code I managed to build until now:

nums = [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]
prime_nums = []
i = 0

#Loop on the array to check each element
while i <= nums.length
  check = 0
  divider = nums[i]

  #While to prevent DivisionByZero error
  while divider != 0
    if nums[i] % divider == 0    #/!\ ERROR HERE
      check  = 1
    end
    divider -= 1
  end
  #Only division by 1 and itself will increment $check
  if check == 2
    prime_nums.append(nums[i])
  end
  i  = 1
end
puts prime_nums

CodePudding user response：

In your example it means that nums[i] returns nil, so the case is that i is greater than 19.

You can change while i <= nums.length to while i < nums.length because arrays are indexed from 0 in Ruby.

CodePudding user response：

undefined method %' for nil:NilClass means you're trying to use % on nil. nil is what you get when you, for example, try to get too much out of an Array.

If we put p "#{i}: #{nums[i]}" in the loop we'll see that i goes to 20. Arrays start counting at 0, so if you have 20 elements in an array the highest index is 19. num[20] is nil.

You can fix this with while i < nums.length to stop at 19.

However, one rarely writes while loops in Ruby. Instead, you iterate using methods. each iterates through each element of an Array. And you can count down with downto.

nums.each do |num|
  check = 0

  num.downto(1) do |divider|
    if num % divider == 0
      check  = 1
    end
  end

  #Only division by 1 and itself will increment $check
  if check == 2
    prime_nums.append(num)
  end
end

Now there's no chance of being off-by-one.