Hi people, I am currently teaching myself C++ with Stroustrup's "Programming, Principles and Practice using C++" and I'm stuck on one of the in-book exercises. It is a chunk of code (~180 lines) of buggy code that, when debugged properly, should create a calculator program. I have reached the limits of my ability and need some more help. There are 8 bugs in total, 5 syntax and 3 logic. I have debugged the syntax errors and now need help with the logic. I think I got one of the logic, so there *should* only be 2 left.
Any help would be very useful. Hopefully the code won't be *too* long.
//
// 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 "stdafx.h"
#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':
{
double val;
cin >> val; // read a floating-point number
return Token('8',val); // let '8' represent "a number"
cin.putback(ch); // put digit back into the input stream
}
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();
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) {
double val;
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;
}
//------------------------------------------------------------------------------
