convert integers to english.

integer_to_english.cpp

#include <iostream>
#include <string>
#include <array>

std::array<std::string, 10> figures{
    "",
    "One ",
    "Two ",
    "Three ",
    "Four ",
    "Five ",
    "Six ",
    "Seven ",
    "Eight ",
    "Nine "
};

std::array<std::string, 10> teens{
    "Ten ",
    "Eleven ",
    "Twelve ",
    "Thirteen ",
    "Fourteen ",
    "Fifteen ",
    "Sixteen ",
    "Seventeen ",
    "Eighteen ",
    "Nineteen "
};

std::array<std::string, 10> tens{
    "",
    "",
    "Twenty ",
    "Thirty ",
    "Forty ",
    "Fifty ",
    "Sixty ",
    "Seventy ",
    "Eighty ",
    "Ninety "
};

std::array<std::string, 4> thousands{
    "",
    "Thousand ",
    "Million ",
    "Billion "
};

std::string convert_hundred(long number)
{
    if(number < 10) return figures[number];
    if(number < 20) return teens[number - 10];

    return 
        tens[number / 10] + 
        figures[number % 10];
}

std::string convert_thousand(long number)
{
    if(number < 100)
        return convert_hundred(number % 100);

    return 
        figures[number / 100] + "Hundred " + 
        convert_hundred(number % 100);
}

std::string convert(std::string number_str)
{
    int length = number_str.size();
    int thousands_idx = length / 3;
    long number = std::atol(number_str.c_str());

    if(number == 0) return "Zero";

    std::string result{};
    int idx{}; 
    while(number)
    {
        std::string current_thousand{
            convert_thousand(number % 1000)
        };

        bool empty {current_thousand.size() == 0}; 
        result = 
            current_thousand + 
            (empty ? "" : thousands[idx]) +
            result;

        number /= 1000;
        idx++;
    }

    result.pop_back();
    return result;
}

int main()
{
    std::cout << convert("1") << "\n";
    std::cout << convert("2") << "\n";
    std::cout << convert("0") << "\n";
    std::cout << convert("14") << "\n";
    std::cout << convert("17") << "\n";
    std::cout << convert("173") << "\n";
    std::cout << convert("273") << "\n";
    std::cout << convert("1234") << "\n";
    std::cout << convert("12345") << "\n";
    std::cout << convert("1000000001") << "\n";
}