Calculator | C++ | Programming Principles and Practice Using C++ : Bjarne Stroustrup

calculator_v2.cpp

// Programming Principles and Practice Using C++ : Bjarne Stroustrup
// Chapter 6 

#include <iostream>
#include <vector>
using namespace std;

// class Token_stream; // member functionss : putback(), get() 
// class Token;        // Token class, member varaibles : kind, value;
double expression();   // Exp = Exp or Exp + Term or Exp - term
double term();         // Term = Term or Term * Primary or Term / Primary
double primary();      // Primary = '(' or Number ; Number = double precision number;
//void error(string);  // Throw errors


//error
void error(string st)
{
    throw runtime_error(st);
}

class Token
{
    public:
        char kind;
        double value;
        // Constructor for operators  
        Token(char ch) 
            :kind(ch){} //empty function.

        //constructor for operands
        Token(char ch, double val) 
            :kind(ch), value(val){}
};

class Token_stream 
{
    private:
        bool full;      // flag for buffer status, empty or full
        Token buffer;   // buffer stores only one token
    public:
        Token_stream() :full(false), buffer(0){};
        Token get();
        void putback(Token t);
    
};

// save the character which we read and is not used by the respeceted function.
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
}

// input character and create Tokens out of it.
Token Token_stream :: get()
{
   if (full){   // check if we have a token already.
       //remove token from buffer
       full = false;
       return buffer;
   } 
   // if buffer is empty
   char ch;      
   cin >> ch;       // reading new token
   switch (ch)
   {
    case ';':
    case 'q':
    case '(':
    case ')':
    case '+':
    case '-':
    case '*':
    case '/':
    case '%':
        return Token(ch);
        break;
    case '.':
    case '0':
    case '1':
    case '2':
    case '3':
    case '4':
    case '5':
    case '6':
    case '7':
    case '8':
    case '9':
        cin.putback(ch); // because we have read a digit (as a character for comparision purpose) which is part of the upcoming stream of digits, so we put it back so we can read it again.
        double val;
        cin >> val;
        return Token('8',val);
        
    default:
        error("Bad Token");
        break;
   }
}

// Token_stream object
Token_stream ts;

// Primary
double primary()
{
    Token t = ts.get();
    switch (t.kind){
        case '(' :
            {
                double d = expression();
                t = ts.get();
                if(t.kind != ')') error("')' expected");
                return d;
            }
        case '8' :
            return t.value;
            break;
        default:
            error("primary expected");
    }

}

// Term
double term()
{
    double left = primary();
    Token t = ts.get();
    while (true){
        switch (t.kind){
            case '*':
                left *= primary();
                t = ts.get();
                break;
            case '/':
            {
                double d = primary();
                if (d == 0) error("divided by zero");
                left /= d;
                t = ts.get();
                break;
            }
            default :
                ts.putback(t);
                return left;
        }
    }
    
}

// Expression
double expression()
{
    double left = term();
    Token t = ts.get();
    while(true){
        switch(t.kind){
            case '+':
                left += term();
                t = ts.get();
                break;
            case '-':
                left -= term();
                t = ts.get();
                break;
            default:
                ts.putback(t);
                return left;
        }
    }
}

// execution starts from here
int main()
{
    double val = 0;
    while(cin){
        Token t = ts.get();
        if (t.kind == 'q') break;                       // Press q to quit
        if (t.kind == ';') cout << "=" << val <<endl;   // Press ; to end expression
        else ts.putback(t);
        val = expression();
    }
}