Operation Manual

Experiments in Python
89
Notes:
Lesson 3.3: Roman Numerals & data manipulation
def int2roman(number):
n u m e r a lO r d e r = [1000,900,500,400,100,90,50,40,10,9,5,4,1]
numeralLetters = {
1000 : "M", 900 : "CM", 500 : "D", 400 : "CD",
100 : "C", 90 : "XC", 50 : "L", 40 : "XL",
10 : "X", 9 : "IX", 5 : "V", 4 : "IV",
1 : "I" }
result = ""
if number < 1 or number > 4999:
raise ValueError
for value in numeralOrder:
while number >= value:
result += numeralLetters[value]
number -= value
return result
try:
print(int2roman(int(input("Enter an integer (1 to 4999): "))))
except ValueError:
print("Try again")
This program introduces the idea of a function. A function groups together
commands into a block. It is the basic method by which you can create your own
Python commands. Functions can have variables given to them and they can give
back an answer once they are finished. They are great for reusing instructions on
different data and getting results. In this case, it will take a normal decimal number
and give back a sequence of letters that are the Roman equivalent.
The following command runs a number of functions all in one:
print(int2roman(int(input("Enter an integer (1 to 4999): "))))
It prints the result of the function “int2roman”. It is started (called) with the result
of “int” and “input”. The “int” function is converting the letters you type into a
number. The “input” function allows Python to get some numbers from the user.
Finally, The characters that the function creates are printed onto the display.
To work out the Roman Numeral equivalent for a number, you just need to keep
taking the largest possible Roman number out of the given number, and adding
the Roman Numeral to the string as it does so. A string is how computer scientists
describe a sequence of characters; letters, numbers and other special symbols
Notice that the “numeralLetters” sequence is made from a pair of numbers and
letters. This is a particular kind of sequence known as a “dictionary”. The order
of the entries in the dictionary is not guaranteed to be the same as you enter it.
This is so the computer can optimise how the data is stored and give you quick
access later. The left part of the dictionary entry is the “key”, and can be almost
anything. To get the value of the entry (the right-hand part), you can ask for it by
the key, using square brackets “[ ]” around the key. The “numeralOrder” is a
normal list, where the order is maintained. This is used to break down the number
coming into the function, from highest to lowest.