# calculator 1.0

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
// Chapter 6 Try This Page 204

#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) { }
};

Token get_token() // read a token from cin
{
char ch;
cin >> ch; // note that >> skips whitespace (space, newline, tab, etc.)

switch (ch) {
//not yet case ‘;’: // for "print"
//not yet 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:
}
}

double expression(); // read and evaluate a Expression
double term(); // read and evaluate a Term

double primary() // read and evaluate a Primary
{
Token t = get_token();
switch (t.kind) {
case ‘(‘: // handle ‘(‘ expression ‘)’
{
double d = expression();
t = get_token();
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");
}
}

int main()
try {
while (cin)
cout << "=" << expression() << ‘\n’;
keep_window_open("~0");
}
catch (exception& e) {
cerr << e.what() << endl;
keep_window_open("~1");
return 1;
}
catch (…) {
cerr << "exception \n";
keep_window_open("~2");
return 2;
}

double expression()
{
double left = term(); // read and evaluate a Term
Token t = get_token(); // get the next token
while (true) {
switch (t.kind) {
case ‘+’:
left += term(); // evaluate Term and add
t = get_token();
break;
case ‘-‘:
left -= term(); // evaluate Term and subtract
t = get_token();
break;
default:
return left; // finally: no more + or -: return the answer
}
}
}

double term()
{
double left = primary();
Token t = get_token(); // get the next token

while (true) {
switch (t.kind) {
case ‘*’:
left *= primary();
t = get_token();
break;
case ‘/’:
{
double d = primary();
if (d == 0) error("divide by zero");
left /= d;
t = get_token();
break;
}
default:
return left;
}
}
}
[/code]

```Output:
4+8
=4
```