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There being only 7 symbols to consider (IVXLCDM), conversion is easily achieved in any number of ways. In general, numerals are formed from left to right, largest value to smallest. However, if a smaller value precedes a larger value, the smaller value is subtracted from the larger value (or is negated). This can lead to problems such as IVX. Reading left to right this would become 10 - ( 5 - 1 ) = 10 - 4 = 6. There's nothing wrong with this, but most people would accept 6 = VI, not IVX.

The problem is there has never been an official standard relating to how Roman numerals are formed. Decimal 1999 could be represented as MCMXCIX or MIM or MDCCCCLXXXXVIIII or even a mixed format like MCMXCVIIII. All are intrinsically correct. However, only the first example conforms to what many would consider to be the "unofficial" standard, whereby certain combinations are no longer permitted (such as IIII, IM and VX).

This standard has been incorporated into the following code.

#include <iostream>

using namespace std;

int main()

{

char roman[11];

int decimal[10];

memset( roman, 0, 11 );

memset( decimal, 0, 10 * sizeof( int ));

cout << endl;

cout << "Enter a Roman number (max. 10 chars from I, V, X, L, C, D or M): ";

cin.getline( roman, 11, '\n' );

strupr( roman ); // convert to uppercase for consistency

// check validity, including all invalid combinations

if( !strlen( roman )

( strspn( roman, "IVXLCDM") != strlen( roman ))

( strstr( roman, "IIII" ))

( strstr( roman, "XXXX" ))

( strstr( roman, "CCCC" ))

( strstr( roman, "MMMM"))

( strstr( roman, "IL" ))

( strstr( roman, "IC" ))

( strstr( roman, "ID" ))

( strstr( roman, "IM" ))

( strstr( roman, "XD" ))

( strstr( roman, "XM" ))

( strstr( roman, "VX" ))

( strstr( roman, "VL" ))

( strstr( roman, "VC" ))

( strstr( roman, "VD" ))

( strstr( roman, "VM" ))

( strstr( roman, "LC" ))

( strstr( roman, "LD" ))

( strstr( roman, "LM" ))

( strstr( roman, "DM" ))

( strstr( roman, "IIV" ))

( strstr( roman, "IIX" ))

( strstr( roman, "XXL" ))

( strstr( roman, "XXC" ))

( strstr( roman, "CCD" ))

( strstr( roman, "CCM" )))

{

cout << roman << " is not a valid roman number." << endl;

return( -1 );

}

// convert to decimal, in reverse order.

int c = 9, total = 0;

while( c >= 0 )

{

switch( roman[c] )

{

case('I'): decimal[c] = 1; break;

case('V'): decimal[c] = 5; break;

case('X'): decimal[c] = 10; break;

case('L'): decimal[c] = 50; break;

case('C'): decimal[c] = 100; break;

case('D'): decimal[c] = 500; break;

case('M'): decimal[c] = 1000; break;

}

if( c < 9 ) // subtraction required?

if( decimal[c] < decimal[c+1] )

decimal[c] *= (-1); // negate

// update total.

total += decimal[c--];

}

cout << "Roman " << roman << " is decimal " << total << endl;

return( 0 );

}

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13y ago

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