Digital Roots

#include <iostream>
#include <string>
using namespace std;
void f(int n)
{ int t,sum=0;
 while(n!=0)
 {t=n%10;
     sum=sum+t;
  n=n/10;}
 if(0<sum&&sum<10)
 {cout<<sum<<endl;
 return;}
else
f(sum);

}

int main()
{string a;
while(cin>>a)
{int i,num=0;
if(a=="0") break;
for(i=0;i<a.length();i++)
num+=a[i]-'0';
f(num);}

 

return 0;}