# calculator 1.1

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Using std_lib_facilities.h by Bjarne Stroustrup.

[code language=”cpp”]
// Philipp Siedler
// Bjarne Stroustrup’s PP

#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()
{
// 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 ‘/’:
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
}
default:
}
}

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
```