Code:

#include <stdio.h>
#include <math.h>
#define MAXNUM 4
int main() {
double input, fraction, integer;
int i, j;
int b10_num[MAXNUM];
char *N_1000[] = {"", "M", "MM"};
char *N_100[] = {"", "C", "CC", "CCC", "CD", "D", "DC", "DCC", "DCCC", "CM"};
char *N_10[] = {"", "X", "XX", "XXX", "XL", "L", "LX", "LXX", "LXXX", "XC"};
char *N_1[] = {"", "I", "II", "III", "IV", "V", "VI", "VII", "VIII", "IX"};
while(1) {
printf("\nEnter a number (between 1 and 3999): ");
scanf("%lf", &input);
if (input < 4000 && input > 0 && modf(input, &integer) == 0)
break;
else {
printf("Error: number is invalid");
continue;
}
}
//transforming into string
/* ERROR LIST
1551 returns 1550
1552 returns 1551
1556 returns 1555
1557 returns 1556
1560 returns 1550
*/
for(i = 0; i < MAXNUM; i++) {
input = input / 10;
printf("input: %g\t", input);
fraction = modf(input, &integer) * 10;
b10_num[i] = (int)fraction;
printf("fraction: %lf\t b10_num[%d]: %d\n", fraction, i, b10_num[i]);
}
input *= pow(10,MAXNUM);
//result
printf("Digits of the number: ");
for(j = 3; j >= 0; j--) {
printf("%d,", b10_num[j]);
}
printf("\nThe number %g as Roman numeral: %s%s%s%s\n", input,
N_1000[b10_num[3]], N_100[b10_num[2]], N_10[b10_num[1]], N_1[b10_num[0]]);
return 0;
}