Roman Numeral in C

Published on 20 October 2019 (Updated: 15 April 2026)
Roman Numeral in C

Welcome to the Roman Numeral in C page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

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

int val[150];

int main(int argc, char **argv)
{
    if (argv[1] == NULL)
    {
        printf("Usage: please provide a string of roman numerals");
        return 0;
    }
    if (strlen(argv[1]) == 0)
    {
        printf("0");
        return 0;
    }

    int len = strlen(argv[1]);
    val['I'] = 1;
    val['V'] = 5;
    val['X'] = 10;
    val['L'] = 50;
    val['C'] = 100;
    val['D'] = 500;
    val['M'] = 1000;
    long long ans = 0;

    for (int i = 1; i < len; ++i)
    {
        if (!val[argv[1][i]])
        {
            printf("Error: invalid string of roman numerals");
            return 0;
        }
        if (val[argv[1][i]] > val[argv[1][i - 1]])
            ans -= 2 * val[argv[1][i - 1]];
        ans += val[argv[1][i]];
    }
    if (!val[argv[1][0]])
    {
        printf("Error: invalid string of roman numerals");
        return 0;
    }
    printf("%lld", ans + val[argv[1][0]]);
}

Roman Numeral in C was written by:

This article was written by:

If you see anything you'd like to change or update, please consider contributing.

Note: The solution shown above is the current solution in the Sample Programs repository as of Apr 15 2026 00:50:56. The solution was first committed on Oct 20 2019 11:59:04. The documentation was last updated on May 15 2023 15:51:23. As a result, documentation below may be outdated.

How to Implement the Solution

Let's understand this code block by block in the order of execution.

Main Function

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

int val[150];

In the first three lines, we are including header files using include directive to utilize some functions defined in header files later in the program. Here, Standard Input/Output header file(<stdio.h>) is called to use printf() function and <string.h> to use strlen() function. strlen() returns the length of string and printf() prints formatted string as output. We are also declaring a variable of length 150.

Now let's move on the main function.

int main(int argc,char **argv)
{
    ...
}

In C, we declare a function using general form:

return_type function_name(parameter){
  ...
}

So, we are declaring main function with return_type integer and argc and argv as parameters to access command line arguments. argc and argv are variables which main function will get when run in command-line. argc stores argument count while argv stores array of strings that are arguments. This should be kept in mind that all command-line arguments are stored as strings.

argv[0] represents first argument which always is equal to name of our program. If we type the following command in terminal:

./roman-numeral IV

Here, ./roman-numeral represents argv[0] and IV represents argv1.

Control Flow

if(argv[1]==NULL){
    printf("Usage: please provide a string of roman numerals");
    return 0;
}

First if condition checks if the argument provided is a null string or not. If so, it prints correct usage pattern and returns with exit code of 0.

if(strlen(argv[1])==0){
    printf("0");
    return 0;
}

Now, if length of the argument provided is 0, then it prints 0 and returns with value 0.

int len=strlen(argv[1]);
val['I']=1;
val['V']=5;
val['X']=10;
val['L']=50;
val['C']=100;
val['D']=500;
val['M']=1000;
long long ans=0;

This blocks initialize a integer variable with the length of string provided by the argument. It also initializes the array. Since, in C we directly do not have dictionaries, here we have utilised the power of ASCII values. "I" has an ASCII value of 73. So, at val[73] = 1. Remaining values also get assigned in the similar fashion. Next 'long long' datatype variable ans is initialized with value of 0.

for(int i=1;i<len;++i){
    if(!val[argv[1][i]]){
        printf("Error: invalid string of roman numerals");
        return 0;
    }
    if(val[argv[1][i]]>val[argv[1][i-1]])ans-=2*val[argv[1][i-1]];
    ans+=val[argv[1][i]];
}

Now, in this for loop, first if conditions check if the each letter in the alphabet is a Roman Numeral. If it is not, it prints Error and exits the main function with exit code 0. Else, it proceeds with the rest of the function. If a smaller value appears before a larger one, the smaller value is subtracted 2 times from the ans. Then, each value is added in the ans.

if(!val[argv[1][0]]){
    printf("Error: invalid string of roman numerals");
    return 0;
}
printf("%lld",ans+val[argv[1][0]]);

Since in the above for loop, we have not considered the value of argv[1][0], we check if it is correct Roman Numeral and prints error message if it is not. Lastly, we print ans + val[argv[1][0]] using the format specifier %lld for long long datatype.

How to Run the Solution

gcc -o roman-numeral roman-numeral.c
./roman-numeral

Another handy option is to compile and run using online C Compiler such as OnlineGDB, Repl