C program that will input a number in digit (up to millions) and output its word equivalent.

hi beloved coders all over the world i just want to have an extra help from you in creating a C program that will input a number in digit (up to millions) and output its word equivalent. Here's what i've got so far.  The code below will output 
its equivalent words  from 0 to 100 but i need 101,102,etc  up to millions to be outputted also. Can you help me continue with the code and if the code below has mistakes can you correct it. Thank you and regards.

#include<stdio.h>
#include<string.h>
main ()
{
    char a[20];
    char *single[] = {"Zero","One","two","three","four","five","six","seven","eight","nine"};
    char *double_digit[] = {"","ten","eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"};
    char *tens_place[] = {"","","twenty","thirty","fourty","fifty","sixty","seventy","eighty","ninety"};
    char *hundred_place[] = {"","","","One hundred","two hundred","three hundred","four hundred","five hundred","six hundred","seven hundred","eight hundred","nine hundred"};
    printf("Enter a number\n");
    scanf("%s",a);
    
    int len = strlen(a);
    int num;
    if(len == 1)
    { num = a[0] - 48;
        printf("%s ",single[num]);
    }
    if(len ==2 && a[0] == 49)
    { num = (a[0] - 48) + (a[1]-48);
        printf("%s ",double_digit[num]);
    }
    else if(len == 2 && a[1] == 48)
    {
      num = (a[0] - 48) + (a[1]-48);
      printf("%s ",tens_place[num]);
    }
    else if(len == 2)
    {
      num = a[0] - 48;
      printf("%s ",tens_place[num]);
      num = a[1] - 48;
      printf("%s ",single[num]);
    }
    else if(len == 3 && a[1] == 48)
    {
      num = (a[1] - 47 ) + (a[0]-47);
        printf("%s ",hundred_place[num]);
    }
    else if(len == 3)
    {
      num = a[0] - 47;
      printf("%s ",hundred_place[num]);
      num = a[0] - 50 ;
      printf("%s ",single[num]);
    }
    }