Convert number to Words

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

// A function that prints given number in words

void convert_to_words(char *num)
{
    // Get number of digits in given number
    int len = strlen(num);

    // Base cases
    if (len == 0)
    {
        fprintf(stderr, "empty string\n");
        return;
    }
    if (len > 4)
    {
        fprintf(stderr, "Length more than 4 is not supported\n");
        return;
    }

    // The first string is not used, it is to make array indexing simple
    char *single_digits[] = {
            "zero", "one", "two", "three", "four",
            "five", "six", "seven", "eight", "nine"
    };

    // The first string is not used, it is to make array indexing simple
    char *two_digits[] = {
            "", "ten", "eleven", "twelve", "thirteen", "fourteen",
            "fifteen", "sixteen", "seventeen", "eighteen", "nineteen"
    };

    // The first two string are not used, they are to make array indexing simple
    char *tens_multiple[] = {
            "", "", "twenty", "thirty", "forty", "fifty",
            "sixty", "seventy", "eighty", "ninety"
    };

    char *tens_power[] = {"hundred", "thousand"};

    // Used for debugging purpose only
    printf("\n%s: ", num);

    // For single digit number
    if (len == 1)
    {
        printf("%s\n", single_digits[*num - '0']);
        return;
    }

    // Iterate while num is not '\0'
    while (*num != '\0')
    {

        // Code path for first 2 digits
        if (len >= 3)
        {
            if (*num - '0' != 0)
            {
                printf("%s ", single_digits[*num - '0']);
                // here len can be 3 or 4
                printf("%s ", tens_power[len - 3]);
            }
            --len;
        }

            // Code path for last 2 digits
        else
        {
            /* Need to explicitly handle 10-19. Sum of the two digits is
               used as index of "two_digits" array of strings */
            if (*num == '1')
            {
                int sum = *num - '0' + *(num + 1) - '0';
                printf("%s\n", two_digits[sum]);
                return;
            }

                // Need to explicitely handle 20
            else if (*num == '2' && *(num + 1) == '0')
            {
                printf("twenty\n");
                return;
            }

                // Rest of the two digit numbers i.e., 21 to 99
            else
            {
                int i = *num - '0';
                printf("%s ", i ? tens_multiple[i] : "");
                ++num;
                if (*num != '0')
                {
                    printf("%s ", single_digits[*num - '0']);
                }
            }
        }
        ++num;
    }
}

int main()
{
    char num[100];
    printf("Enter number to convert to words : ");
    scanf("%s", num);
    convert_to_words(num);

    return 0;
}

OUTPUT

Enter number to convert to words : 1684

1684: one thousand six hundred eighty four