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Conversion Calculator

ProgrammerXYZ (23)
I am supposed to be creating a program that converts roman numbers to its decimal values. I have done something, but the math is wrong. When entering MCMLXXVIII, I am supposed to get 1978, and instead I get 2178. I am using substrings to solve this problem, but I am not getting anywhere. Can somebody please advise me on what to do?

Thanks 

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  #include<iomanip>
#include<iostream>
#include<string>
using namespace std;

int main(){


	int M = 1000;
	int D = 500;
	int C = 100;
	int L = 50;
	int X = 10;
	int V = 5;
	int I = 1;
	

	int num = 0;

	cout << " Enter the Roman Numeral Value: ";
	string roman;
	cin >> roman;

	string sub = roman.substr(0, 2);
	cout << sub << endl;
	




	for (int i = 0; i < roman.length(); i++)
	{
		
		switch (roman.at(i))
		{
		
		case 'M':
		case 'm':
			num += M;
			break;
		case 'D':
		case 'd':
			num += D;
			break;
		case 'C':
		case 'c':
			num += C;
			break;
		case 'L':
		case 'l':
			num += L;
			break;
		case 'X':
		case 'x':
			num += X;
			break;
		case 'V':
		case 'v':
			num += V;
			break;
		case 'I':
		case 'i':
			num += I;
			break;
		}
	}

	cout << num << endl;
}
C0D3FR3AK (61)
Here you go, this is what you COULD do. 

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#include<iomanip>
#include<iostream>
#include<string>
using namespace std;

const int I = 1;       
const int V = 5;
const int X = 10;
const int L = 50;
const int C = 100;
const int D = 500;       
const int M = 1000;
const int bad_val = -1; 
 
     
// This function Takes the number and changes it into
// a Roman Numeral.
char Romandigit ( int val )
{
   char digit_char;
   if ( val == I )
     digit_char = 'I';
   if ( val == V )
     digit_char = 'V';
   if ( val == X )
     digit_char = 'X';
   if ( val == L )
     digit_char = 'L';
   if ( val == C )
     digit_char = 'C';
   if ( val == D )
     digit_char = 'D';
   if ( val == M )
     digit_char = 'M';
  return digit_char;
}

//This function Takes the Roman Numeral and changes it into a 
//number and it also tests to find if a bad value is 
//present.
int Roman_digit_Value( char roman_digit )
{
   int val;
   if ( roman_digit == 'I' )
      val = I;
   else if ( roman_digit == 'V' )
      val = V;
   else if ( roman_digit == 'X' )
      val = X;
   else if ( roman_digit == 'L' )
      val = L;
   else if ( roman_digit == 'C' )
      val = C;
   else if ( roman_digit == 'D' )
      val = D;
   else if ( roman_digit == 'M' )
      val = M;
   else 
       val = bad_val;
   return val;
}

//This function prints the roman numeral as it reads the smallest
//amount of Roman Numerals possible.
void Print_Roman_Numeral ( int val )
{
   int number = val;
   int high_digit = M;
      while ( number > 0 )
      {
      if ( number > high_digit )
         cout << Roman_digit_Value ( high_digit );
         number = number - high_digit;
      if ( number > V )
         cout << Roman_digit_Value ( V );
         number = number - V;
      if ( number > X )
         cout << Roman_digit_Value ( X );
         number = number - X;
      if ( number > L )
         cout << Roman_digit_Value ( L );
         number = number - L;
      if ( number > C )
         cout << Roman_digit_Value ( C );
         number = number - C;
      if ( number > D )
         cout << Roman_digit_Value ( D );
         number = number - D;
      if ( number > M )
         cout << Roman_digit_Value ( M );
         number = number - M;
      }
}
//This function reads the input from the input file
//and then changes it to values.
void Print_Result ( int Roman_Num, int Second_Roman_Num, int Result, char operation )
{
   
   if ( operation == '+' )
      Print_Roman_Numeral ( Result );
      cout << "The sum of " << Roman_Num << " and " << Second_Roman_Num << 
      " is " << "(" << Result << ")" << endl; 
   if ( operation == '-' )
      cout << "The difference of " << Roman_Num << " and " << Second_Roman_Num << 
      " is " << "(" << Result << ")"; 
   if ( operation == '/' )
      cout << "The quotient of " << Roman_Num << " and " << Second_Roman_Num << 
      " is " << "(" << Result << ")";
   if ( operation == '*' )
      cout << "The quotient of " << Roman_Num << " and " << Second_Roman_Num << 
      " is " << "(" << Result << ")"; 
}
//This function turns the two romman numeral numbers
//into one by doing the specified operation.
int Return_Operation ( int Roman_Num, int Second_Roman_Num, char operation )
{
   int Result = 0;
   if ( operation == '+' )
      Result = Roman_Num + Second_Roman_Num;
   if ( operation == '-' )
      Result = Roman_Num - Second_Roman_Num;
   if ( operation == '/' )
      Result = Roman_Num / Second_Roman_Num;
   if ( operation == '*' )
      Result = Roman_Num * Second_Roman_Num;
   return Result;
}      
//This function takes the roman digits thet were inputted
//and then changes them to a number value and it 
//determines weaher the value is bad.
int Get_Roman_Numeral()
{
   int num = 0;
   char ch = 0;
   int count = 1;
  
   while (count <= 7 ) 
   {
   cin >> ch;
 
   int value = Roman_digit_Value(ch);
       num = num + value;
   count++;
   }
   return num;  
}
       
int main()
{ 
  char operation = '+';
  string input_Roman;  
  int Roman_Num = 0;
  int Second_Roman_Num = 0;
  int Result = 0;
  Roman_Num = Get_Roman_Numeral();
      while ( !cin.eof() ) 
      {   
         cout << "The first number is " << Roman_Num << endl;
         Second_Roman_Num = Get_Roman_Numeral();
         cout << "The second number is " << Second_Roman_Num << endl;
         cin >> operation;
         cout << "Arithmetic operation is " << operation << endl;
         Result = Return_Operation (Second_Roman_Num, Roman_Num, operation);
         Print_Result( Roman_Num, Second_Roman_Num, Result, operation );
         Roman_Num = Get_Roman_Numeral();

      }
      return 0;
}
Last edited on
whitenite1 (1785)
@ProgrammerXYZ

Your main problem in the Roman Numeral to decimal conversion program, is you're not checking if a number is to be subtracted from a larger number. If the value of a Roman Numeral is lower than the next value, that value gets subtracted. You are just checking the values, and adding.

Value .. MCMLXXVIII
M = 1000
C is less than the next M, so 100 is subtracted from the 1000 = 900
L = 50
X = 10
X = 10
V = 5
I = 1
I = 1
I = 1

Total = 1978
ProgrammerXYZ (23)
Yeah I know, but I tried everything, and the program doesnt do what its supposed to. The value doesnt come out to 1978. I dont know how to check if the number is larger than the previous number using substrings and void function, as that is what I am supposed to do
ProgrammerXYZ (23)
@C0D3FR3AK

Your idea works, but the math ends up being wrong and thats the main problem. Thanks though
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