//
// This is example code from Chapter 6.5.2.3 "Expressions: Third time lucky" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

//------------------------------------------------------------------------------

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) { }
};

//------------------------------------------------------------------------------

Token get_token();    // read a token from cin

//------------------------------------------------------------------------------

double term();        // read and evaluate a Term

//------------------------------------------------------------------------------

double expression()
{
    double left = term();      // read and evaluate a Term
    Token t = get_token();     // get the next token
    while(true) {    
        switch(t.kind) {
        case '+':
            left += term();    // evaluate Term and add
            t = get_token();
            break;
        case '-':
            left -= term();    // evaluate Term and subtract
            t = get_token();
            break;
        default:
            return left;       // finally: no more + or -: return the answer
        }
    }
}

//------------------------------------------------------------------------------