Пример #1
0
double expression(TokenStream& tokenStream)
{
    double result = term(tokenStream);       // set result to value of term
    Token token = tokenStream.getToken();  // get next token from tokenStream

    while (true)
    {
        if (token.kind == '+')
        {
            result += term(tokenStream);      // evaluate term and add to result
            token = tokenStream.getToken(); // get next token from tokenStream
        }

        if (token.kind == '-')
        {
            result -= term(tokenStream);      // evaluate term and subtract from result
            token = tokenStream.getToken(); // get next token from tokenStream
        }

        if (token.kind != '+' && token.kind != '-')
        {
            tokenStream.setToken(token);    // reset token into token stream
            return result;                    // return result
        }
    }
}
Пример #2
0
void calculate()
{
    const char quit = 'q';       // t.kind==quit means that t is a quit Token
    const char print = ';';      // t.kind==print means that t is a print Token
    const std::string prompt = "> ";
    const std::string equals = "= ";  // used to indicate that what follows is an evaluation
    TokenStream tokenStream;
    std::string enterToClose = "~~";

    while (std::cin)
        try
        {
            std::cout << prompt;
            Token token = tokenStream.getToken();

            if (token.kind == print)
                token = tokenStream.getToken();   // eat ';'

            if (token.kind == quit)
                break;

            tokenStream.setToken(token);      // reset token into tokenStream
            std::cout << equals << expression(tokenStream) << '\n';
        }

        catch (std::exception& e)
        {
            std::cerr << e.what() << '\n';                      // write error message
            tokenStream.ignore(print);
        }
}
Пример #3
0
double term(TokenStream& tokenStream)
{
    double result = primary(tokenStream);   // set result to value of primary
    Token token = tokenStream.getToken(); // get next token from tokenStream

    while (true)
    {
        if (token.kind == '*')
        {
            result *= primary(tokenStream);   // evaluate primary and multiply with result
            token = tokenStream.getToken(); // get next token from tokenStream
        }

        else if (token.kind == '/')
        {
            double divisor = primary(tokenStream);     // set divisor to value of primary
            if (divisor == 0) error("divide by zero"); // check for error
            result /= divisor;                           // divide result by divisor
            token = tokenStream.getToken();      // get next token from tokenStream
        }

        else if (token.kind == '%') // modulo will only work on ints for this implementation
        {
            int numer = result; // cast result to int
            int denom = primary(tokenStream); // cast denom to int
            if (denom == 0) error("%: divide by zero"); // error check
            result = numer % denom; // get result
            token = tokenStream.getToken();      // get next token from tokenStream
        }

        else
        {
            tokenStream.setToken(token);   // put token back into the Token stream
            return result;                 // return result
        }
    }
}