Integer to roman numeral
========================

.. contents:: :local:

.. include:: roman_numeral_introduction.rst


Exercise
--------


Approach
^^^^^^^^

Create a ``dict`` with the 7 single-letter and the 6 double-letter symbols sorted with *decending* values. Iterate over the ``dict`` and 

.. interactive_code_block:: 
   :caption: Complete the function that converts an integer to a roman numeral

   def integer_to_roman_numeral(integer):
       """Convert an integer to a roman numeral."""
       # Create a dict with the roman symbols and their integer values
       roman_symbols = {                'M':1000,
           'CM':900, 'D':500, 'CD':400, 'C': 100, 
           'XC': 90, 'L': 50, 'XL': 40, 'X':  10,
           'IX':  9, 'V':  5, 'IV':  4, 'I':   1 }
       # Create a variable to be able to add the roman symbols
       roman_numeral = ''
       # Substract the values of the roman symbols as long as the integer is greater then 0
       while #..........
           # Iterate over the dict with the roman symbols
           for #..........
               # Calculate how many times the roman symbol has to be added
               repetitions = #..........
               # Add the symbol(s) to the result
               roman_numeral += #..........
               # Substract the values from the integer
               integer -= #..........

       return roman_numeral


   # Create a dict with test values
   test_values = {'CXXIII': 123, 'CCCXXI':321, 'CDXLIV':444, 'CMXCIX': 999, 'MMXXI': 2021}

   # Print a table with test values and the result of the function
   print('   Test values  | Result')
   print('----------------+-------')
   for roman_numeral, integer in test_values.items():
       print('{:6} | {:>6} | {:>6}'.format(integer, roman_numeral, integer_to_roman_numeral(integer)))


Know how
^^^^^^^^

Iterating over a dict
"""""""""""""""""""""

Though the data type of a ``dict`` is a *mapping* and not a *sequence* like ``str``, ``tuple`` and ``list`` it can be seen as a sequence of key / value pairs. The most noticable difference between a mapping and a sequence is that a ``dict``'s value can *only* be accessed by its key and not by its index. Since Python 3.7 the order is even predictable (new elements will be appended at the end).

.. interactive_code_block:: 
   :caption: Iterate over the keys of the dict

   roman_symbols = {                'M':1000,
       'CM':900, 'D':500, 'CD':400, 'C': 100, 
       'XC': 90, 'L': 50, 'XL': 40, 'X':  10,
       'IX':  9, 'V':  5, 'IV':  4, 'I':   1 }

   # Iterate over the keys of the dict
   for roman_symbol in roman_symbols:
       print(roman_symbol)

Since the solution above only returns the keys of the dict an extra step is required to get the values as well.

.. interactive_code_block:: 
   :caption: Iterate over the keys of the dict and get the values

   roman_symbols = {                'M':1000,
       'CM':900, 'D':500, 'CD':400, 'C': 100, 
       'XC': 90, 'L': 50, 'XL': 40, 'X':  10,
       'IX':  9, 'V':  5, 'IV':  4, 'I':   1 }
    
   # Iterate over the keys of the dict
   for roman_symbol in roman_symbols:
       # Get the value for the key
       value = roman_symbols[roman_symbol]
       print(roman_symbol, value)

An alternative solution is to use the ``.items()`` method of the ``dict`` class.

.. interactive_code_block:: 
   :caption: Iterate over the keys and values of the dict

   roman_symbols = {                'M':1000,
       'CM':900, 'D':500, 'CD':400, 'C': 100, 
       'XC': 90, 'L': 50, 'XL': 40, 'X':  10,
       'IX':  9, 'V':  5, 'IV':  4, 'I':   1 }
   
   # Iterate over the keys and values of the dict    
   for roman_symbol, value in roman_symbols.items():
       print(roman_symbol, value)

Go to :ref:`solution <pwp-i2rn-ex1>`.