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Converting Roman Numerals to Decimal lying between 1 to 3999
• Difficulty Level : Medium
• Last Updated : 10 May, 2021

Given a Romal numeral, the task is to find its corresponding decimal value.

Example :

```Input: IX
Output: 9
IX is a Roman symbol which represents 9

Input: XL
Output: 40
XL is a Roman symbol which represents 40

Input: MCMIV
Output: 1904
M is a thousand,
CM is nine hundred and
IV is four```

Roman numerals are based on the following symbols.

```SYMBOL       VALUE
I            1
IV           4
V            5
IX           9
X            10
XL           40
L            50
XC           90
C            100
CD           400
D            500
CM           900
M            1000```

Approach: A number in Roman Numerals is a string of these symbols written in descending order(e.g. M’s first, followed by D’s, etc.). However, in a few specific cases, to avoid four characters being repeated in succession(such as IIII or XXXX), subtractive notation is often used as follows:

• I placed before V or X indicates one less, so four is IV (one less than 5) and 9 is IX (one less than 10).
• X placed before L or C indicates ten less, so forty is XL (10 less than 50) and 90 is XC (ten less than a hundred).
• C placed before D or M indicates a hundred less, so four hundred is CD (a hundred less than five hundred) and nine hundred is CM (a hundred less than a thousand).

Algorithm to convert Roman Numerals to Integer Number:

1. Split the Roman Numeral string into Roman Symbols (character).
2. Convert each symbol of Roman Numerals into the value it represents.
3. Take symbol one by one from starting from index 0:
1. If current value of symbol is greater than or equal to the value of next symbol, then add this value to the running total.
2. else subtract this value by adding the value of next symbol to the running total.

Following is the implementation of the above algorithm:

## C++

 `// Program to convert Roman``// Numerals to Numbers``#include ``using` `namespace` `std;` `// This function returns value``// of a Roman symbol``int` `value(``char` `r)``{``    ``if` `(r == ``'I'``)``        ``return` `1;``    ``if` `(r == ``'V'``)``        ``return` `5;``    ``if` `(r == ``'X'``)``        ``return` `10;``    ``if` `(r == ``'L'``)``        ``return` `50;``    ``if` `(r == ``'C'``)``        ``return` `100;``    ``if` `(r == ``'D'``)``        ``return` `500;``    ``if` `(r == ``'M'``)``        ``return` `1000;` `    ``return` `-1;``}` `// Returns decimal value of``// roman numaral``int` `romanToDecimal(string& str)``{``    ``// Initialize result``    ``int` `res = 0;` `    ``// Traverse given input``    ``for` `(``int` `i = 0; i < str.length(); i++)``    ``{``        ``// Getting value of symbol s[i]``        ``int` `s1 = value(str[i]);` `        ``if` `(i + 1 < str.length())``        ``{``            ``// Getting value of symbol s[i+1]``            ``int` `s2 = value(str[i + 1]);` `            ``// Comparing both values``            ``if` `(s1 >= s2)``            ``{``                ``// Value of current symbol``                ``// is greater or equal to``                ``// the next symbol``                ``res = res + s1;``            ``}``            ``else``            ``{``                ``// Value of current symbol is``                ``// less than the next symbol``                ``res = res + s2 - s1;``                ``i++;``            ``}``        ``}``        ``else` `{``            ``res = res + s1;``        ``}``    ``}``    ``return` `res;``}` `// Driver Code``int` `main()``{``    ``// Considering inputs given are valid``    ``string str = ``"MCMIV"``;``    ``cout << ``"Integer form of Roman Numeral is "``        ``<< romanToDecimal(str) << endl;` `    ``return` `0;``}`

## Java

 `// Program to convert Roman``// Numerals to Numbers``import` `java.util.*;` `public` `class` `RomanToNumber``{``    ``// This function returns``    ``// value of a Roman symbol``    ``int` `value(``char` `r)``    ``{``        ``if` `(r == ``'I'``)``            ``return` `1``;``        ``if` `(r == ``'V'``)``            ``return` `5``;``        ``if` `(r == ``'X'``)``            ``return` `10``;``        ``if` `(r == ``'L'``)``            ``return` `50``;``        ``if` `(r == ``'C'``)``            ``return` `100``;``        ``if` `(r == ``'D'``)``            ``return` `500``;``        ``if` `(r == ``'M'``)``            ``return` `1000``;``        ``return` `-``1``;``    ``}` `    ``// Finds decimal value of a``    ``// given romal numeral``    ``int` `romanToDecimal(String str)``    ``{``        ``// Initialize result``        ``int` `res = ``0``;` `        ``for` `(``int` `i = ``0``; i < str.length(); i++)``        ``{``            ``// Getting value of symbol s[i]``            ``int` `s1 = value(str.charAt(i));` `            ``// Getting value of symbol s[i+1]``            ``if` `(i + ``1` `< str.length())``            ``{``                ``int` `s2 = value(str.charAt(i + ``1``));` `                ``// Comparing both values``                ``if` `(s1 >= s2)``                ``{``                    ``// Value of current symbol``                    ``// is greater or equalto``                    ``// the next symbol``                    ``res = res + s1;``                ``}``                ``else``                ``{``                    ``// Value of current symbol is``                    ``// less than the next symbol``                    ``res = res + s2 - s1;``                    ``i++;``                ``}``            ``}``            ``else` `{``                ``res = res + s1;``            ``}``        ``}` `        ``return` `res;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `main(String args[])``    ``{``        ``RomanToNumber ob = ``new` `RomanToNumber();` `        ``// Considering inputs given are valid``        ``String str = ``"MCMIV"``;``        ``System.out.println(``"Integer form of Roman Numeral"``                           ``+ ``" is "``                           ``+ ob.romanToDecimal(str));``    ``}``}`

## Python

 `# Python program to convert Roman Numerals``# to Numbers` `# This function returns value of each Roman symbol`  `def` `value(r):``    ``if` `(r ``=``=` `'I'``):``        ``return` `1``    ``if` `(r ``=``=` `'V'``):``        ``return` `5``    ``if` `(r ``=``=` `'X'``):``        ``return` `10``    ``if` `(r ``=``=` `'L'``):``        ``return` `50``    ``if` `(r ``=``=` `'C'``):``        ``return` `100``    ``if` `(r ``=``=` `'D'``):``        ``return` `500``    ``if` `(r ``=``=` `'M'``):``        ``return` `1000``    ``return` `-``1`  `def` `romanToDecimal(``str``):``    ``res ``=` `0``    ``i ``=` `0` `    ``while` `(i < ``len``(``str``)):` `        ``# Getting value of symbol s[i]``        ``s1 ``=` `value(``str``[i])` `        ``if` `(i ``+` `1` `< ``len``(``str``)):` `            ``# Getting value of symbol s[i + 1]``            ``s2 ``=` `value(``str``[i ``+` `1``])` `            ``# Comparing both values``            ``if` `(s1 >``=` `s2):` `                ``# Value of current symbol is greater``                ``# or equal to the next symbol``                ``res ``=` `res ``+` `s1``                ``i ``=` `i ``+` `1``            ``else``:` `                ``# Value of current symbol is greater``                ``# or equal to the next symbol``                ``res ``=` `res ``+` `s2 ``-` `s1``                ``i ``=` `i ``+` `2``        ``else``:``            ``res ``=` `res ``+` `s1``            ``i ``=` `i ``+` `1` `    ``return` `res`  `# Driver code``print``(``"Integer form of Roman Numeral is"``),``print``(romanToDecimal(``"MCMIV"``))`

## C#

 `// C# Program to convert Roman``// Numerals to Numbers``using` `System;` `class` `GFG {``    ``// This function returns value``    ``// of a Roman symbol``    ``public` `virtual` `int` `value(``char` `r)``    ``{``        ``if` `(r == ``'I'``)``            ``return` `1;``        ``if` `(r == ``'V'``)``            ``return` `5;``        ``if` `(r == ``'X'``)``            ``return` `10;``        ``if` `(r == ``'L'``)``            ``return` `50;``        ``if` `(r == ``'C'``)``            ``return` `100;``        ``if` `(r == ``'D'``)``            ``return` `500;``        ``if` `(r == ``'M'``)``            ``return` `1000;``        ``return` `-1;``    ``}` `    ``// Finds decimal value of a``    ``// given romal numeral``    ``public` `virtual` `int` `romanToDecimal(``string` `str)``    ``{``        ``// Initialize result``        ``int` `res = 0;` `        ``for` `(``int` `i = 0; i < str.Length; i++)``        ``{``            ``// Getting value of symbol s[i]``            ``int` `s1 = value(str[i]);` `            ``// Getting value of symbol s[i+1]``            ``if` `(i + 1 < str.Length)``            ``{``                ``int` `s2 = value(str[i + 1]);` `                ``// Comparing both values``                ``if` `(s1 >= s2)``                ``{``                    ``// Value of current symbol is greater``                    ``// or equalto the next symbol``                    ``res = res + s1;``                ``}``                ``else``                ``{``                    ``res = res + s2 - s1;``                    ``i++; ``// Value of current symbol is``                    ``// less than the next symbol``                ``}``            ``}``            ``else` `{``                ``res = res + s1;``                ``i++;``            ``}``        ``}` `        ``return` `res;``    ``}` `    ``// Driver Code``    ``public` `static` `void` `Main(``string``[] args)``    ``{``        ``GFG ob = ``new` `GFG();` `        ``// Considering inputs given are valid``        ``string` `str = ``"MCMIV"``;``        ``Console.WriteLine(``"Integer form of Roman Numeral"``                          ``+ ``" is "``                          ``+ ob.romanToDecimal(str));``    ``}``}` `// This code is contributed by Shrikant13`

## PHP

 `= ``\$s2``)``            ``{``                ``// Value of current symbol``                ``// is greater or equal to``                ``// the next symbol``                ``\$res` `= ``\$res` `+ ``\$s1``;``            ``}``            ``else``            ``{``                ``\$res` `= ``\$res` `+ ``\$s2` `- ``\$s1``;``                ``\$i``++; ``// Value of current symbol is``                      ``// less than the next symbol``            ``}``        ``}``        ``else``        ``{``            ``\$res` `= ``\$res` `+ ``\$s1``;``            ``\$i``++;``        ``}``    ``}``    ``return` `\$res``;``}` `// Driver Code` `// Considering inputs``// given are valid``\$str` `=``"MCMIV"``;``echo` `"Integer form of Roman Numeral is "``,``              ``romanToDecimal(``\$str``), ``"\n"``;` `// This code is contributed by ajit``?>`

## Python3

 `def` `romanToInt(rom):``    ``value ``=` `{``        ``'M'``: ``1000``,``        ``'D'``: ``500``,``        ``'C'``: ``100``,``        ``'L'``: ``50``,``        ``'X'``: ``10``,``        ``'V'``: ``5``,``        ``'I'``: ``1``    ``}` `    ``# Initialize previous character and answer``    ``p ``=` `0``    ``ans ``=` `0` `    ``# Traverse through all characters``    ``n ``=` `len``(rom)``    ``for` `i ``in` `range``(n``-``1``, ``-``1``, ``-``1``):` `        ``# If greater than or equal to previous,``        ``# add to answer``        ``if` `value[rom[i]] >``=` `p:``            ``ans ``+``=` `value[rom[i]]` `        ``# If smaller than previous``        ``else``:``            ``ans ``-``=` `value[rom[i]]` `        ``# Update previous``        ``p ``=` `value[rom[i]]` `    ``print``(ans)`  `romanToInt(``'MCMIV'``)`

## Javascript

 ``
Output
`Integer form of Roman Numeral is 1904`

Complexity Analysis:

• Time Complexity: O(n), where n is the length of the string.
Only one traversal of the string is required.
• Space Complexity: O(1).
As no extra space is required.

Another solution –

## C++

 `// Program to convert Roman``// Numerals to Numbers``#include ``using` `namespace` `std;` `// This function returns value``// of a Roman symbol``int` `romanToDecimal(string& str)``{``    ``map<``char``, ``int``> m;``    ``m.insert({ ``'I'``, 1 });``    ``m.insert({ ``'V'``, 5 });``    ``m.insert({ ``'X'``, 10 });``    ``m.insert({ ``'L'``, 50 });``    ``m.insert({ ``'C'``, 100 });``    ``m.insert({ ``'D'``, 500 });``    ``m.insert({ ``'M'``, 1000 });``    ``int` `sum = 0;``    ``for` `(``int` `i = 0; i < str.length(); i++)``    ``{``        ``/*If present value is less than next value,``          ``subtract present from next value and add the``          ``resultant to the sum variable.*/``        ``if` `(m[str[i]] < m[str[i + 1]])``        ``{``            ``sum+=m[str[i+1]]-m[str[i]];``            ``i++;``            ``continue``;``        ``}``        ``sum += m[str[i]];``    ``}``    ``return` `sum;``}` `// Driver Code``int` `main()``{``    ``// Considering inputs given are valid``    ``string str = ``"MCMIV"``;``    ``cout << ``"Integer form of Roman Numeral is "``         ``<< romanToDecimal(str) << endl;` `    ``return` `0;``}`

## Java

 `// Program to convert Roman``// Numerals to Numbers``import` `java.util.Map;``import` `java.util.HashMap;` `class` `GFG{``    ` `private` `static` `final` `Map roman = ``new` `HashMap()``{{``    ``put(``'I'``, ``1``);``    ``put(``'V'``, ``5``);``    ``put(``'X'``, ``10``);``    ``put(``'L'``, ``50``);``    ``put(``'C'``, ``100``);``    ``put(``'D'``, ``500``);``    ``put(``'M'``, ``1000``);``}};` `// This function returns value``// of a Roman symbol``private` `static` `int` `romanToInt(String s)``{``    ``int` `sum = ``0``;``    ``int` `n = s.length();``    ` `    ``for``(``int` `i = ``0``; i < n; i++)``    ``{``        ` `        ``// If present value is less than next value,``        ``// subtract present from next value and add the``        ``// resultant to the sum variable.``        ``if` `(i != n - ``1` `&& roman.get(s.charAt(i)) <``                          ``roman.get(s.charAt(i + ``1``)))``        ``{``            ``sum += roman.get(s.charAt(i + ``1``)) -``                   ``roman.get(s.charAt(i));``            ``i++;``        ``}``        ``else``        ``{``            ``sum += roman.get(s.charAt(i));``        ``}``    ``}``    ``return` `sum;``}` `// Driver Code``public` `static` `void` `main(String[] args)``{``    ` `      ``// Considering inputs given are valid``    ``String input = ``"MCMIV"``;``    ` `    ``System.out.print(``"Integer form of Roman Numeral is "` `+``                     ``romanToInt(input));``}``}` `// This code is contributed by rahuldevgarg`

Output
`Integer form of Roman Numeral is 1904`

Time complexity – O(N)
Auxiliary Space – O(1)

References: https://en.wikipedia.org/wiki/Roman_numerals
This article is contributed by Rahul Agrawal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.