How to verify total digits and ensure characters are not entered?

Question

in C++

string phone_number

If asking for a 10 digit phone number for example, how would you ensure 10 digits are entered?  Also, how would you ensure no characters were input by mistake.  I am also curious how to ensure the user doesn't enter past the prompt by mistake (leaving it empty).

Answer 1

#include <iostream>
#include <string>
#include <cctype>

using namespace std;

bool isValidPhone(const string& s) {
    if (s.empty()) return false;
    if (s.size() != 10) return false;

    for (char ch : s) {
        if (!isdigit(static_cast<unsigned char>(ch)))
            return false;
    }
    return true;
}

int main() {
    string phone_number;

    while (true) {
        cout << "Enter a 10-digit phone number: ";
        getline(cin, phone_number);   // allows empty input too (we can detect it)

        if (isValidPhone(phone_number)) {
            break;
        }

        cout << "Invalid input. Please enter exactly 10 digits (0-9) only.\n";
    }

    cout << "Valid phone number: " << phone_number << endl;
    return 0;
}

Answer 2

#include <iostream>
#include <string>
#include <cctype>

bool isValidPhoneNumber(const std::string& phone)
{
    // Check length
    if (phone.length() != 10)
        return false;

    // Check all characters are digits
    for (char c : phone)
    {
        if (!std::isdigit(c))
            return false;
    }

    return true;
}

int main()
{
    std::string phone_number;

    while (true)
    {
        std::cout << "Enter a 10-digit phone number: ";
        std::getline(std::cin, phone_number);

        // Prevent empty input (user just pressed Enter)
        if (phone_number.empty())
        {
            std::cout << "Input cannot be empty.\n";
            continue;
        }

        if (isValidPhoneNumber(phone_number))
        {
            break;  // valid input
        }

        std::cout << "Invalid phone number. Please enter exactly 10 digits.\n";
    }

    std::cout << "Phone number accepted: " << phone_number << "\n";
}

Answer 3

#include <iostream>
#include <string>
#include <cctype>
#include <algorithm>

bool isAllDigits(const std::string &s) {
    for (unsigned char ch : s) {
        if (!std::isdigit(ch)) return false;
    }
    return true;
}

std::string trim(const std::string &s) {
    auto first = s.find_first_not_of(" \t\r\n");
    if (first == std::string::npos) return "";
    auto last = s.find_last_not_of(" \t\r\n");
    return s.substr(first, last - first + 1);
}

int main() {
    std::string phone;
    while (true) {
        std::cout << "Enter a 10-digit phone number: ";
        std::getline(std::cin, phone);

        // Trim whitespace so "   " counts as empty and " 1234567890 " is accepted
        phone = trim(phone);

        if (phone.empty()) {
            std::cout << "Input cannot be empty. Please enter the phone number.\n";
            continue;
        }

        if (phone.size() != 10) {
            std::cout << "Invalid length. Please enter exactly 10 digits.\n";
            continue;
        }

        if (!isAllDigits(phone)) {
            std::cout << "Invalid characters detected. Use digits 0-9 only.\n";
            continue;
        }

        // If we reach here, phone is valid
        break;
    }

    std::cout << "Phone number accepted: " << phone << '\n';
    return 0;
}