Frequency of the Digits in the given string

I cracked this recently in hackerrank and all the tests passed. So, I wanted to record this, and this is the most frequently asked question in interviews.  Here, I also mentioned another important trick of converting the string to integer without using any library functions. 

How to convert string to an integer? or conversion of char[] to int[] ?

if (num[i] >= '0' && num[i] <= '9') {
    //printf("DEBUG: index %d num %c value %d\n", i, num[i], (num[i] - '0'));
    result[i] = (num[i] - '0');

    //printf("DEBUG: result %d\n", result[i]);
}

#include <stdio.h>

#include <string.h>

#include <math.h>

#include <stdlib.h>

#define MAX_SIZE 1000

#define TOTAL_DIGITS 10

int main() {

    char num[MAX_SIZE] = {'\0'};

    int len, i = 0;

    long int index = 0;

    int result[TOTAL_DIGITS] = {0};

    

    /* Enter your code here. Read input from STDIN. Print output to STDOUT */

    scanf("%s", num);

    len = strlen(num);

    //printf("\nThe frequency for the string to be found is %s length %d\n", num, len);

     

    for (i = 0; i < len; i++) {

        /* Requirement is to find frequency of only digits, 

           so skipping the alphabets */

        if (num[i] >= '0' && num[i] <= '9') {

            //printf("DEBUG: index %d num %c value %d\n", i, num[i], (num[i] - '0'));

            result[(num[i] - '0')]++;

            /* If you are uncommenting the below line, remove the above line. */

            //printf("DEBUG: result %d\n", result[(num[i] - '0')]++);    

        }

    }    

    for (i = 0; i < TOTAL_DIGITS; i++) {

        printf("%d ", result[i]);   

    }

    return 0;

}

Output

Compiler Message

Success

Input (stdin)

• a11472o5t6

Expected Output

• 0 2 1 0 1 1 1 1 0 0