parse digit

#include <iostream>
#include <fstream>
#include <string>

using namespace std;

int main () {
    string line, element;
    ifstream myfile("input.txt");
    size_t old_index, new_index;
    bool is_digit;

    if (myfile.is_open()) {
        while ( getline(myfile, line) ) {
            new_index = -1;

            do {
                old_index = new_index + 1;
                new_index = line.find_first_of(" ", new_index + 1);

                element = line.substr(old_index, new_index - old_index);

                is_digit = true;
                for (int i = element.length(); i != 0; i--) {
                    if (!isdigit(element[i])) {
                        is_digit = false;
                    }
                }

                if (is_digit) {
                    cout << "'" << element << "' is digit" << endl;
                } else {
                    cout << "'" << element << "' is not digit" << endl;
                }
            } while (new_index != string::npos);
        }

        myfile.close();
    } else {
        cout << "Unable to open file";
    }

    return 0;
}