is_num.cpp

#include <iostream>
#include <set>
#include <map>

bool
is_num(std::string& string_token, std::set<char>& accepted_characters){
    auto accepted_chars = std::map<char,int>();
    for(const auto& character : string_token) {
        if(accepted_characters.find(character) == accepted_characters.end()) {
            if(character < '0' or character > '9') {
                accepted_chars[character]++;
            }
        }
    }

    for(const auto& char_count_pairs : accepted_chars) {
        if(char_count_pairs.second > 1) {
            return false;
        }
    }
    return true;
}

int 
main() {
    /* for this function, you need to make a set of characters to compare the strings against */
    auto holder = std::string("0123456789.-Ee");
    auto accepted_char_set = std::set<char>();
    for(auto& ch : holder) {
        accepted_char_set.insert(ch);
    }
    /* this is where you  call the function using the set of characters as well as
     * the token string
     * */
    auto demo = std::string("demo");
    is_num(demo, accepted_char_set);

    return 0;
}