ar2rom - Convert Arabic numerals to Roman numerals

ar2rom.c

/**
 * ar2rom - Convert Arabic numerals to Roman numerals
 * Copyright (C) 2013 Matous J. Fialka, <http://mjf.cz/>
 * Released under the terms of The MIT License
 *
 * Compile: cc -pedantic -ansi -Wall -Werror -o ar2rom ar2rom.c
 */

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

int
main(int argc, char *argv[])
{
	int arg;
	long left;

	if(argc < 2) {
		printf("usage: ar2rom [ARNUM ...]\n");
		return 0;
	}
	for(arg = 1; arg < argc; arg++) {
		left = atol(argv[arg]);
		printf("%-16ld\t", left);
		if(left < 1) {
			printf("NaN\n");
			continue;
		}
		while(left >= 1000)
			left -= 1000, printf("M");
		if(left >= 900) {
			left -= 900;
			printf("CM");
		}
		if(left >= 500) {
			left -= 500;
			printf("D");
		}
		if(left >= 400) {
			left -= 400;
			printf("CD");
		}
		while(left >= 100) {
			left -= 100;
			printf("C");
		}
		if(left >= 50) {
			left -= 50;
			printf("L");
		}
		if(left >= 40) {
			left -= 40;
			printf("XL");
		}
		while(left >= 10) {
			left -= 10;
			printf("X");
		}
		if(left >= 9) {
			left -= 9;
			printf("IX");
		}
		if(left >= 5) {
			left -= 5;
			printf("V");
		}
		if(left >= 4) {
			left -= 4;
			printf("IV");
		}
		while(left >= 1) {
			left -= 1;
			printf("I");
		}
		printf("\n");
	}
	return 0;
}

Example:

$ ar2rom 1979 2013 9999 -123
1979                    MCMLXXIX
2013                    MMXIII
9999                    MMMMMMMMMCMLXLIX
-123                    NaN