My very simple guessing game

game.cc

#include <iostream>
#include <fstream>
#include <vector>

// check if input is a letter
bool isLetter(char character)
{
    char alphabet[52] = {'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z'};
    for(short i = 0; i < sizeof(alphabet); ++i)
        if(tolower(character) == alphabet[i]) return true;
    return false;
}

// check if a letter has been used or not
bool usedChar(char letter)
{
    letter = tolower(letter);
    static std::vector<char> vec{};
    for(char a : vec)
        if(a == letter) return true;
    vec.push_back(letter);
    return false;
}

// the game
int main(const int argc, const char* const* argv) noexcept
{
    const int maxFails = 5;
    int curFails = 0;

    std::string word = "";
    for(int i = 1; i < argc; ++i)
    {
        word += std::string(argv[i]);
        if(i < argc) word += ' ';
    }

    std::string prog = "";
    for(int i = 0; i < word.size(); ++i)
        if(isLetter(word.at(i)))
            prog += '_';
        else
            prog += ' ';

    if(prog == word) 
    {
        std::cout << "Pass the word via the command-line" << std::endl; 
        return 0;
    }

    std::cout << "You get " << maxFails << " failed attempts before you lose!" << std::endl;

    while(prog != word)
    {
        std::cout << "You have: " << prog << std::endl;
        std::string userInput = "";
promptUI:
        while(userInput.size() <= 0 || !isLetter(userInput.at(0)))
        {
            std::cout << "Please enter a letter: ";
            getline(std::cin, userInput);
        }
        if(usedChar(userInput.at(0)))
        {
            userInput = "";
            std::cout << "That letter has already been used." << std::endl;
            goto promptUI;
        }
        bool wasUsed = false;
        for(int i = 0; i < word.size(); ++i)
            if(tolower(word.at(i)) == tolower(userInput.at(0)))
            {
                prog.at(i) = word.at(i);
                wasUsed = true;
            }
        if(wasUsed)
            std::cout << "Yup! That was a letter!" << std::endl;
        else 
        {
            std::cout << "Nope! Wasn't a letter. Try again." << std::endl;
            if(maxFails > 0)
                if(++curFails < maxFails)
                    std::cout << "You have " << (maxFails - curFails) << " fails left!" << std::endl;
                else
                {
                    std::cout << "You ran out of fails! Here was the answer:" << std::endl << word << std::endl;
                    return 0;
                }
                
        }
        std::cout << std::endl;
    }
    std::cout << "Answer: " << prog << std::endl;
    std::cout << "Yay! You got it!" << std::endl;
    return 0;
}