What is the algorithm for conversion between Roman numerals and decimal?

Answer 1

I can't speak about C, but I imagine that it is similar process to Visual Basic or HyperCard.

Roman numerals are not "place dependent" so you need to write a repeating loop which will cycle through the individual characters of the string and add their value to a variable, 100 for C, 10 for X and so on, and at the end of the loop display the variable. Start the variable with a value of 0 and then it will increase as you process each character.

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Not strictly true, as using the above method IV would give 6 instead of 4.

You would need to loop from right to left, adding up the individual characters.

But if an individual character has a lower value than the character to its right, you would need to subtract instead of add.

e.g.

XIV = 5 - 1 + 10 = 14

MCM = 1000 - 100 + 1000 = 1900

XVII = 1 + 1 + 5 + 10 = 17

Answer 2

To convert Roman numerals to decimal, start from left to right and compare each symbol to the one that follows it. If the current symbol is smaller than the next symbol, subtract its value from the total. Otherwise, add its value to the total. To convert decimal to Roman numerals, repeatedly subtract the largest possible Roman numeral value from the decimal number until it reaches 0, building the Roman numeral representation as you go.