Calculator

[Fauch911]

#include <iostream>
#include <limits>
#include <stdexcept>

using namespace std;

//declaring classes

class token
{
    public:
        char kind;
        double value;
        token (char ch)
            :kind(ch), value(0) {}
        token (char ch, double val)
            :kind(ch), value(val) {}
};

class token_stream
{
    public:
        token_stream();
        token token_get();
        void putback(token t);
    private:
        bool full;
        token buffer;
};

// class member functions of token_stream

token_stream::token_stream()
:full(false), buffer(0) {}

void token_stream::putback(token t)
{
    if (full==true) throw runtime_error ("putback into a full buffer");
    buffer = t;
    full=true;
}

token token_stream::token_get()
{
    if (full) {
        full=false;
        return buffer;
    }

    char ch;
    cin >> ch;

    switch (ch) {
    case ';':
    case 'q':
    case '(': case ')': case '+': case '-': case '*': case '/': case '{': case '}': case'!':
        return token(ch);

    case '.':
    case '0': case '1': case '2': case '3': case '4':
    case '5': case '6': case '7': case '8': case '9':
        {
            cin.putback(ch);
            double val;
            cin >> val;
            return token ('8',val);
        }
    default:
        throw runtime_error("bad token");
    }
}

//declaring functions

token_stream ts;
double expression();
double term();
double primary();
int factorial(int number);

//functions



double primary()
{
    token t = ts.token_get();
    switch (t.kind) {

    case '{':
    {
        double d = expression();
        t = ts.token_get();
        if (t.kind != '}') throw runtime_error ("']' expected");
        return d;
    }

    case '(':
    {
     double d = expression();
     t = ts.token_get();
     if (t.kind != ')') throw runtime_error("')' expected");
     return d;
    }

     case '!':
     {
        t = ts.token_get();
        double d = factorial(t.value);
        return d;
     }

    case '8':
        return t.value;

    case 'q':
        cout << "exit";
        return 0;

    default:
        throw runtime_error("primary expected");


    }
}

double term()
{
    double left = primary();
    token t = ts.token_get();

    while (true) {
        switch (t.kind) {

        case '*':
        left *= primary();
        t = ts.token_get();
        break;

        case '/':
            {
                double d = primary();
                if (d==0) throw runtime_error("divide by zero");
                left /=d;
                t = ts.token_get();
                break;
            }

        default:
            ts.putback(t);
            return left;
        }
    }
}


double expression()
{
    double left = term();
    token t = ts.token_get();

    while(true) {
        switch(t.kind) {

        case '+':
            left+=term();
            t = ts.token_get();
            break;

        case '-':
            left-=term();
            t = ts.token_get();
            break;

        default:
            ts.putback(t);
            return left;
            }
    }
}

int factorial(int number)
{
    int solution = 1;
    for (int i=number; i>1; i--) {
        solution= solution*i;
    }
    return solution;
}




int main()
try
{
        cout << "A simple calculator able to deal with +,-,*,/. Enter ';' to calculate and 'q' to quit" << '\n';
        cout << ">";
         //main loop
        while (cin) {
            token t = ts.token_get();

            if (t.kind == 'q') {
                cout << "Thanks for using the calculator" << '\n';
                break; //'q' for quit
            }

            if (t.kind == ';') // ';' for "print now"
                cout << "=" << expression() << '\n'<< ">";

            else {
                ts.putback(t);
                cout << "=" << expression() << '\n' << ">";
            }
        }
}
catch (exception& e) {
    cerr << "error: " << e.what() << '\n';
    return 1;
}
catch (...) {
    cerr << "Unknown exception!\n";
    return 2;
}

Hello guys,

I'm working on Sir Stroustrups Beginner book. This is, as you can see, a simple calculator.
It is supposed to exit on pressing 'q', which it does, but only if I input 'q' before I do anything else. Afterwards I can't exit the program that way anymore.

From my understanding the following happens (Please correct me if I'm wrong)

- due to line 212, my last token is put back into the stream, thus the program calls "expression()" which calls "term()" which calls "primary()". Now I tried fixing the problem by inserting case "q" into primary, but from my understanding I can not end the program from a function, I can only "return" to main();

How please do I fix this? Feel free to criticize the rest of the code as well (keep in mind, I'm a beginner)

[kbw]

The app is stuck in the parser at that point. You need to change line 118, (case 'q' in primary()) to call exit(0) rather than return 0.

[nomijigr]

here is the answer to what you are looking for...interrupt...

http://www.drdobbs.com/implementing-interrupt-service-routines/184401485

[Fauch911]

Thanks for the answers.