Convert an int to a Roman numeral

pip3 search roman-numerals
roman-numerals (0.1.0)               - A library to convert between integers
                                       and Unicode Roman numerals.

toroman.py

Please type an integer in the range 1 to 4999 inclusive: 2019
2019 = MMXIX

Please type an integer in the range 1 to 4999 inclusive: 1968
1968 = MCMLXVIII

Please type an integer in the range 1 to 4999 inclusive:

Things to try

1. Change the four lists

   thousands = ["", "M", "MM", "MMM", "MMMM"]
   hundreds  = ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"]
   tens      = ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"]
   ones      = ["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"]
   

   to the following list of four lists. See “nest lists” in Lists. Note that
   len(rows) is 4, because rows is a list containing 4 items.
   len(rows[0]) is 10, because rows[0] is a list of 10 items.
   len(rows[0][0]) is 0 because rows[0][0] is the string "" of 0 characters.

   rows = [
       ["", "M", "MM", "MMM", "MMMM"],
       ["", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"],
       ["", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"],
       ["", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"]
   ]
   

   Change the definition of the toRoman function from

   def toRoman(n):
       "Convert one integer to a Roman numeral."
       assert isinstance(n, int) and 1 <= n <= 4999
   
       d = f"{n:04}"    #d is a string of 4 digits.
   
       i = int(d[0])    #d[0] is the leftmost digit.
       s = thousands[i]
   
       i = int(d[1])    #d[1] is the next-to-leftmost digit.
       s += hundreds[i] #means s = s + hundreds[i]
   
       i = int(d[2])    #d[2] is the third-from-leftmost digit.
       s += tens[i]
   
       i = int(d[3])    #d[3] is the fourth-from-leftmost digit.
       s += ones[i]
   
       return s
   

   to

   def toRoman(n):
       "Convert one integer to a Roman numeral."
       assert isinstance(n, int) and 1 <= n <= 4999
   
       d = f"{n:04}"    #d is a string of 4 digits.
   
       i = int(d[0])
       s = rows[0][i]
   
       i = int(d[1])
       s += rows[1][i]  #means s = s + rows[1][i]
   
       i = int(d[2])
       s += rows[2][i]
   
       i = int(d[3])
       s += rows[3][i]
   
       return s
   

   and then to

   def toRoman(n):
       "Convert one integer to a Roman numeral."
       assert isinstance(n, int) and 1 <= n <= 4999
   
       d = f"{n:04}"    #d is a string of 4 digits.
       s = ""
   
       for r, c in enumerate(d): #r is the row number: 0, 1, 2, 3
           i = int(c)
           s = s + rows[r][i]    #or s += rows[r][i]
   
       return s
   

   or even to the following. t is a tuple containing two items. The integer t[0] is what we just called r, and the one-character string t[1] is what we just called c.

   import functools
   

   def toRoman(n):
       "Convert one integer to a Roman numeral."
       assert isinstance(n, int) and 1 <= n <= 4999
   
       d = f"{n:04}"    #d is a string of 4 digits.
       return functools.reduce(lambda s, t: s + rows[t[0]][int(t[1])], enumerate(d), "")
   

2. Convert a Roman numeral to an int.

   def fromRoman(s):
       "Convert a Roman numeral to an integer."
       assert isinstance(s, str)
   
       n = 0
       place = 1000
   
       for row in rows:
           prefixes = [prefix for prefix in row if s.startswith(prefix)]
           longestPrefix = max(prefixes, key = len)
           i = row.index(longestPrefix)
           n += place * i               #means n = n + place * i
           s = s[len(longestPrefix):]   #Chop longestPrefix off the front of s
           place //= 10                 #means place = place // 10
   
       if s != "":
           raise ValueError(f'"{s}" is not a Roman numeral.')
       return n