Console decimal to hexadecimal converter. (0123456789 to 0123456789ABCDEF)

decimal_to_hexadecimal.cpp

#include <iostream>
#include <vector>

using std::cin, std::cout, std::endl, std::vector, std::string, std::atoi;

char single_hex(int dec);

bool is_whole_number(string s);

long long get_input();

int main() {
    cout << "Console decimal to hexadecimal converter. (0123456789 to 0123456789ABCDEF)\n";

    long long input = get_input();

    vector<char> output;

    long long last = input;
    while (true) {
        output.push_back(single_hex(input % 16));
        input /= 16;

        if (input == last) {
            int start = output.size() - 1;
            if (output.size() > 1 && output[start] == '0') start--;

            for (int i = start; i >= 0; i--) {
                cout << output[i];
            }
            cout << endl;

            return 0;
        } else {
            last = input;
        }
    }
}

char single_hex(int dec) {
    dec %= 16;

    switch (dec) {
        case 0:
            return '0';
        case 1:
            return '1';
        case 2:
            return '2';
        case 3:
            return '3';
        case 4:
            return '4';
        case 5:
            return '5';
        case 6:
            return '6';
        case 7:
            return '7';
        case 8:
            return '8';
        case 9:
            return '9';
        case 10:
            return 'a';
        case 11:
            return 'b';
        case 12:
            return 'c';
        case 13:
            return 'd';
        case 14:
            return 'e';
        case 15:
            return 'f';
    }

    return '\0'; //prevents compiler warning.
}

bool is_whole_number(string s) {
    for (int i = 0; i < s.length(); i++) {
        if (s[i] == '-') continue;
        if (!isdigit(s[i])) return false;
    }

    return true;
}

long long get_input() {
    cout << "Input an integer for conversion: ";

    validate:

    string sinput;
    cin >> sinput;

    while (!is_whole_number(sinput)) {
        cout << "Input must be an integer!\nInput an integer for conversion: ";
        cin >> sinput;
    }

    long long input = atoll(sinput.c_str());

    if (input < 0) {
        cout << "Input must be greater than 0!\nInput a new integer for conversion: ";
        goto validate;
    }

    return input;
}