# calculator 1.0

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

//	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) { }
};

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 '/':
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:
}
}

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;
}
}
}
Output:
4+8
=4

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