C program to convert roman numbers to decimal numbers

Roman numerals are a numeral system that originated in ancient Rome and remained the usual way of writing numbers throughout Europe. In C programming, converting roman numbers to decimal requires understanding the basic roman symbols and their subtraction rule.

Roman Numeral System

The basic roman numerals and their decimal values are −

Conversion Rule

The key rule for roman to decimal conversion is −

• If a smaller numeral appears after a larger one, add it (VI = 5+1 = 6)
• If a smaller numeral appears before a larger one, subtract it (IV = 5-1 = 4)

Algorithm

The conversion algorithm works as follows −

1. Convert each roman character to its decimal equivalent
2. Compare each value with the next value
3. If current value >= next value, add it to result
4. If current value < next value, subtract it from result

Example: Complete Program

#include <stdio.h>
#include <string.h>

int main() {
    char roman[30];
    int deci = 0;
    int length, i, d[30];
    
    printf("Roman to Decimal Conversion<br>");
    printf("==========================<br>");
    printf("Roman Numerals: I=1, V=5, X=10, L=50, C=100, D=500, M=1000<br><br>");
    printf("Enter a Roman numeral: ");
    scanf("%s", roman);
    
    length = strlen(roman);
    
    /* Convert each roman character to decimal */
    for(i = 0; i < length; i++) {
        switch(roman[i]) {
            case 'm':
            case 'M': d[i] = 1000; break;
            case 'd':
            case 'D': d[i] = 500; break;
            case 'c':
            case 'C': d[i] = 100; break;
            case 'l':
            case 'L': d[i] = 50; break;
            case 'x':
            case 'X': d[i] = 10; break;
            case 'v':
            case 'V': d[i] = 5; break;
            case 'i':
            case 'I': d[i] = 1; break;
            default: 
                printf("Invalid roman numeral!<br>");
                return 1;
        }
    }
    
    /* Apply addition/subtraction rule */
    for(i = 0; i < length; i++) {
        if(i == length-1 || d[i] >= d[i+1])
            deci += d[i];
        else
            deci -= d[i];
    }
    
    printf("The decimal equivalent of '%s' is: %d<br>", roman, deci);
    return 0;
}

Roman to Decimal Conversion
==========================
Roman Numerals: I=1, V=5, X=10, L=50, C=100, D=500, M=1000

Enter a Roman numeral: MCMLIV
The decimal equivalent of 'MCMLIV' is: 1954

How It Works

For the input "MCMLIV" (1954) −

• M = 1000, next is C (100) ? 1000 ? 100, so add: 1000
• C = 100, next is M (1000) ? 100 < 1000, so subtract: -100
• M = 1000, next is L (50) ? 1000 ? 50, so add: 1000
• L = 50, next is I (1) ? 50 ? 1, so add: 50
• I = 1, next is V (5) ? 1 < 5, so subtract: -1
• V = 5, last character ? add: 5
• Result: 1000 - 100 + 1000 + 50 - 1 + 5 = 1954

Conclusion

Converting roman numerals to decimal in C involves mapping each roman character to its value and applying the subtraction rule. The algorithm handles both uppercase and lowercase roman numerals correctly.