calculator 1.2

// Philipp Siedler
// Bjarne Stroustrup's PP
// Chapter 6 Try This Page 216

#include "std_lib_facilities.h"

class Token
{
    public:
        char kind; // what kind of token
        double value; // for numbers: a value
        Token(char ch) // make a Token from a char
        :kind(ch), value(0) { }
        Token(char ch, double val) // make a Token from a char and a double
        :kind(ch), value(val) { }
};

class Token_stream
{
    public:
        Token_stream(); // make a Token_stream that reads from cin
        Token get(); // get a Token (get() is defined elsewhere)
        void putback(Token t); // put a Token back
    private:
        bool full; // is there a Token in the buffer?
        Token buffer; // here is where we keep a Token put back using putback()
};

// The constructor just sets full to indicate that the buffer is empty:
Token_stream::Token_stream()
:full(false), buffer(0) // no Token in buffer
{
}

// The putback() member function puts its argument back into the Token_stream's buffer:
void Token_stream::putback(Token t)
{
    if (full) error("putback() into a full buffer");
    buffer = t; // copy t to buffer
    full = true; // buffer is now full
}

Token Token_stream::get()
{
    if (full)
    { // do we already have a Token ready?
      // remove token from buffer
        full = false;
        return buffer;
    }

    char ch;
    cin & gt; > ch; // note that >> skips whitespace (space, newline, tab, etc.)

    switch (ch)
    {
        case ';': // for "print"
        case 'q': // for "quit"
        case '(':
        case ')':
        case '+':
        case '-':
        case '*':
        case '/':
            return Token{ ch }; // let each character represent itself
        case '.':
        case '0':
        case '1':
        case '2':
        case '3':
        case '4':
        case '5':
        case '6':
        case '7':
        case '8':
        case '9':
            {
                cin.putback(ch); // put digit back into the input stream
                double val;
                cin & gt; > val; // read a floating-point number
                return Token{ '8', val }; // let '8' represent "a number"
            }
        default:
            error("Bad token");
    }
}

Token_stream ts; // provides get() and putback()
double expression(); // declaration so that primary() can call expression()

// deal with numbers and parentheses
double primary()
{
    Token t = ts.get();
    switch (t.kind)
    {
        case '(': // handle '(' expression ')'
            {
                double d = expression();
                t = ts.get();
                if (t.kind != ')') error("')' expected");
                return d;
            }
        case '8': // we use '8' to represent a number
            return t.value; // return the number's value
        default:
            error("primary expected");
    }
}

// deal with *, /, and %
double term()
{
    double left = primary();
    Token t = ts.get(); // get the next token from token stream

    while (true)
    {
        switch (t.kind)
        {
            case '*':
                left *= primary();
                t = ts.get();
                break;
            case '/':
                {
                    double d = primary();
                    if (d == 0) error("divide by zero");
                    left /= d;
                    t = ts.get();
                    break;
                }
            default:
                ts.putback(t); // put t back into the token stream
                return left;
        }
    }
}

// deal with + and -
double expression()
{
    double left = term(); // read and evaluate a Term
    Token t = ts.get(); // get the next token from token stream

    while (true)
    {
        switch (t.kind)
        {
            case '+':
                left += term(); // evaluate Term and add
                t = ts.get();
                break;
            case '-':
                left -= term(); // evaluate Term and subtract
                t = ts.get();
                break;
            default:
                ts.putback(t); // put t back into the token stream
                return left; // finally: no more + or -: return the answer
        }
    }
}

int main()
try
{
    /* First Version of 6.7
    while (cin)
    cout << "=" << expression() << '\n';
    keep_window_open("~0");
    */

    double val = 0;
    while (cin)
    {
        Token t = ts.get();

        if (t.kind == 'q') break; // 'q' for quit
        if (t.kind == ';') // ';' for "print now"
            cout & lt; < "=" & lt; < val & lt; < '\n';
else
            ts.putback(t);
        val = expression();
    }
}
catch (exception& e) {
    cerr & lt; < "error: " & lt; < e.what() & lt; < '\n';
    return 1;
}
catch (...) {
    cerr & lt; < "Oops: unknown exception!\n";
    return 2;
}