Converting Roman Numerals to Decimal In C++

January 4, 2022

Convert Roman Numerals to Decimal

Today in this article we will be discussing how to Convert Roman Numerals to Decimal.

Lets understand this with the help of example :- 

  • Input:- IX

  • Output:- 9 

IX is a Roman symbol which represents 9

Convert Sentence into equivalent mobile

Algorithm

  • First we’ll take an unordered_map that will map the character to the corresponding numerical value .
  • Will iterate from right to left throughout the string starting from i = size()-2 to  i>=0;
  • After that we will check if the mp[s[i]]<mp[s[i+1]] if this is true then will decrement the value of mp[ s[i]] from ans 
  • Otherwise we will add the  value of mp[ s[i]] in ans
  • Outside the loop we will return the ans variable that will contain the numerical value of given roman string
Convert Roman Numerals to Decimal

C++ Code :-

#include <bits/stdc++.h>
using namespace std;

int romanToInt(string s) {

  //unordered map that will store the value of roman character
  unordered_map <char,int> mp = { {'I',1}, {'V',5}, {'X',10}, {'L',50}, {'C',100}, {'D',500}, {'M',1000} };
  int ans=mp[s.back()];
  for(int i=s.size()-2;i>=0;i--){

    if(mp[s[i]]<mp[s[i+1]])
      ans-=mp[s[i]];

    else
     ans+=mp[s[i]];
  }
  return ans;
}

int main()
{ 
//string str that will be passed to the function romanToInt too convert it into integer
  string str="XI";
  cout<<romanToInt(str);
   return 0;
}

Output:-

11