How toscope a variable in a function.

[mattbrad83]

OK. here is my problem. I am having a hard time understanding how to scope. I am trying to fix the code listed below. I am getting an error at line 188 when it tries to access val. what I am not getting is how I tell it to look in the scope of Token to find val.

//
// This is example code from Chapter 6.7 "Trying the second version" of
// "Software - Principles and Practice using C++" by Bjarne Stroustrup
//

/*
	This file is known as calculator02buggy.cpp

	I have inserted 5 errors that should cause this not to compile
	I have inserted 3 logic errors that should cause the program to give wrong results

	First try to find an remove the bugs without looking in the book.
	If that gets tedious, compare the code to that in the book (or posted source code)

	Happy hunting!

*/

#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 >> 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 '9':
        {    
            cin.putback(ch);         // put digit back into the input stream
            double val;
            cin >> 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\n");
            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();
        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
{
    while (cin) {
        Token t = ts.get();

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

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

[cire]

We can see that val is assigned the return value of expression on line 191, so val should share the value of the return type (ie. double.) All that remains is to define the variable in the correct scope. The obvious place to put such a definition would be right before the while loop begins (between lines 182 and 183.)

I don't see how the Token type is involved at all here.

[TheIdeasMan]

Hi,

It looks to me that t is of type Token , which has a public member value, so to access that you need t.value as in:

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

So the rules for accessing info from a class:

1. Make a variable in main of the type of the class (An Object) (line 184);
2. Public member variables can be accessed with the Object variable name, the dot operator, and the member variable name, as in: t.value;
3. Public class member functions can be called through an object . If a MyTokenFunction existed, then one could call it with t.MyTokenFunction() . The function can take arguments and return a value, which should be assigned to a variable of a compatible type.

Hope all goes well :+)

Edit:
I hadn't seen cire post, and there are several things going on here.