int to binary

#include <iostream>
#include <string>

      int num[10] = {0,1,2,3,4,5,6,7,8,9};
const int bits    = 5;
const int max     = 1<<bits;

std::string  tobits (int i) {
        std::string s="";
        char ch;
        int b=1;
        while (b<max) {
                if (i & b) ch='1'; else ch='0';
                s=ch+s;
                b<<=1;
        }
        return s;
}

int main() {