Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 6 Try This Page 208
Using std_lib_facilities.h by Bjarne Stroustrup.
[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP
// Chapter 6 Try This Page 208
#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 ‘8’: 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");
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();
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
{
/* First Version of 6.7
while (cin)
cout << "=" << expression() << ‘\n’;
keep_window_open("~0");
*/
double val = 0;
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();
}
}
catch (exception& e) {
cerr << "error: " << e.what() << ‘\n’;
return 1;
}
catch (…) {
cerr << "Oops: unknown exception!\n";
return 2;
}
[/code]
Output: 4 + 5 ; =9 3 + 1; =4
