Digital Roots

 1 #include<stdio.h>
 2 #include<string.h>
 3 int fun(int n)
 4 {
 5  int sum=0;
 6     do
 7     {
 8        sum=0;
 9        while(n>0)
10        {
11          sum+=n%10;
12          n/=10;
13        }
14        n=sum;
15     }
16     while(n>=10);
17     return sum;
18 }
19 int main()
20 {
21     int sum,i,len;
22     char s[1100];
23     while(scanf("%s",s))
24     {
25         if(strcmp(s,"0")==0)
26             break;
27         else
28            len=strlen(s);
29            sum=0;
30            for(i=0;i<len;i++)
31             sum+=s[i]-‘0‘;
32            while(sum>=10)
33             {
34              sum=fun(sum);
35             }
36              printf("%d\n",sum);
37     }
38     return 0;
39 }