Question

I am new to Ruby and I am trying to write a program which convert Roman numerals to numbers.

This is what I did so far:

roman_numbers = {"M" => 1000, "D" => 500, "C" => 100, "L" => 50, "X" => 10, "V" => 5, "I" => 1}
number_by_user = "MCMXCIX"
singlenum = number_by_user.split(//).reverse!

l = singlenum.length
result =0


result = roman_numbers[singlenum[0]]
puts result
for i in 0..l-1
  if roman_numbers.key?(singlenum[i])
    **if (roman_numbers[singlenum[i]] > roman_numbers[singlenum[i+1]])** #gives error
      result = result - roman_numbers[singlenum[i+1]]
    elsif (roman_numbers[singlenum[i]]== roman_numbers[singlenum[i+1]] || **roman_numbers[singlenum[i]] < roman_numbers[singlenum[i+1]])** #gives error
      result = result + roman_numbers[singlenum[i+1]]
    end
    puts roman_numbers[singlenum[i]]
  else
    puts "One of the values are not roman"
    break
  end  
end
puts "The number is: " , result

but it gives me the following error (see the line with comment):

:in `>': comparison of Fixnum with nil failed (ArgumentError)

Answer 1

You have an off-by-one error. Arrays are 0 indexed. Your singlenum.length in this case is 7, but in your for loop, you go up to 6, then try to reference singlenum[7] to compare to singlenum[6]. singlenum[7] is nil, so it doesn’t understand the < operator.