http://www.stroustrup.com/Programming/PPP2code/std_lib_facilities.h
My version is spelt differently so adjust the code accordingly if copying and pasting.
Chapter 6 // Drill 1
Take the calculator from the file calculator02buggy.cpp. Get it to compile. You need to find and fix a few bugs. Those bugs are not inserted in the text in the book. Find the three logic errors deviously inserted in calculator02buggy.cpp and remove them so that the calculator produces correct results.
#include "stdafx.h"
#include "std_lib_facilities_new_version.h"
using namespace std;
//
// 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 and 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!
*/
double val; // messy way for main to access val in get()
//------------------------------------------------------------------------------
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
Token_stream::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 '8': case '9':
{
cin.putback(ch); // put digit back into the input stream
cin >> val; // read a floating-point number
return Token('#', 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)");
return d;
break;
}
case '#': // we use '#' to represent a number
{
return t.value; // return the number's value
break;
}
default:
error("primary expected");
}
}
//------------------------------------------------------------------------------
// deal with *, /
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();
break;
}
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;
}
//------------------------------------------------------------------------------
The five errors were pretty easy to catch as visual studio lights them up in the same way as a spelling mistake on word. The only one I had trouble rectifying was the variable val which stores the returned values that have been worked out. Eventually I just gave up trying to get the compiler to look in the right function and just made it a global variable so it can be accessed from anywhere but I'm pretty sure that isn't the answer.
The logic errors were quite obvious as well, although I'm not sure if there were 4 or 3. He says 3 but I found 4.
1. Multiplication doesn't work. To fix this I added parenthesis to the block of code in the switch statement. It was also missing the 'break;'
2. The subtracting was actually adding, that was a simple fix in the code to simply change + to -.
3. The number 8 was missing in the case list for the switch statement found in Token_stream::get(). I also changed the 'kind' for a number from '8' to '#' to make things less confusing.
4. I'm not sure about this one but in his code, the function which deals with * and /, it also says %. Now I tried to add some code so it could deal with modulo but it wouldn't work so I brushed it off as a typo seeing as though he said there were only 3 logic errors that would cause the program to give incorrect results.
As far as I am aware, the above code does give correct results.
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