# calculator 3.0 [allow underscores]

Bjarne Stroustrup “Programming Principles and Practice Using C++”
Chapter 7 Exercise 1
Using std_lib_facilities.h by Bjarne Stroustrup.

Exercise 1. Allow underscores in the calculator’s variable names.

```//	Philipp Siedler
//	Bjarne Stroustrup's PP
//	Chapter 7 Exercise 1

#include "std_lib_facilities.h"

struct Token {
char kind;
double value;
string name;
Token(char ch) :kind(ch), value(0) { }
Token(char ch, double val) :kind(ch), value(val) { }
Token(char ch, string val) :kind(ch), name(val) { }
};

class Token_stream {
bool full;
Token buffer;
public:
Token_stream() :full(0), buffer(0) { }
Token get();
void unget(Token t) { buffer = t; full = true; }
void ignore(char);
};

const char let = '#';
const char exitprog = 'E';
const char print = ';';
const char number = '8';
const char name = 'a';
const char sqroot = 'S';
const char power = 'P';

Token Token_stream::get()
{
if (full) {
full = false; return buffer;
}
char ch;
cin >> ch;
switch (ch) {
case '(':
case ')':
case '+':
case '-':
case '*':
case '/':
case '%':
case ';':
case '=':
case 'k':
case ',':
{
}
case '#':
{
}
case '.':
case '0':
case '1':
case '2':
case '3':
case '4':
case '5':
case '6':
case '7':
case '8':
case '9':
{
cin.unget();
double val;
cin >> val;
}
default:
{
if (isalpha(ch) || ch == '_') { //is ch a letter?
string s;
s += ch;
while (cin.get(ch) && (isalpha(ch) || isdigit(ch) || ch == '_')) { //reads chars, strings or digits
s += ch; //Error 2: s = ch;
}
cin.unget(); //puts the most recently read character back into the stream
}
}
}
}

void Token_stream::ignore(char c)
{
if (full && c == buffer.kind) {
full = false;
return;
}
full = false;
char ch;
while (cin >> ch) {
if (ch == c) return;
}
}

struct Variable {
string name;
double value;
Variable(string n, double v) :name(n), value(v) { }
};

vector<Variable> names;

double get_value(string s)
{
for (int i = 0; i < names.size(); ++i) {
if (names[i].name == s) {
return names[i].value;
}
}
error("get: undefined name ", s);
return 0.0;
}

void set_value(string s, double d)
{
for (int i = 0; i <= names.size(); ++i) {
if (names[i].name == s) {
names[i].value = d;
return;
}
}
error("set: undefined name ", s);
}

bool is_declared(string s)
{
for (int i = 0; i<names.size(); ++i) {
if (names[i].name == s) return true;
}
return false;
}

Token_stream ts;

double expression();

double primary()
{
Token t = ts.get();
switch (t.kind) {
case '(':
{
double d = expression();
t = ts.get();
if (t.kind != ')') error("')' expected");
return d;
}
case '-':
{
return -primary();
}
case '+':
{
return primary();
}
case number:
{
return t.value;
}
case sqroot:
{
t = ts.get();
if (t.kind != '(') {
error("'(' expected");
}
ts.unget(t);
double d = primary();
if (d < 0.0) {
error("negative square root");
}
return sqrt(d);
}
case power:
{
t = ts.get();
if (t.kind != '(') {
error("'(' expected");
}

double x = int(expression());

t = ts.get();
if (t.kind != ',') {
error("',' expected");
}

int n = int(expression());

t = ts.get();
if (t.kind == ')') {
return pow(x, n);
}
else {
error("Expected ')'");
}
}
case name:
{
return get_value(t.name);
}
default:
{
error("primary expected");
return 0.0;
}
}
}

double term()
{
double left = primary();
while (true) {
Token t = ts.get();
switch (t.kind) {
case 'k':
{
left *= 1000;
break;
}
case '*':
{
left *= primary();
break;
}
case '/':
{
double d = primary();
if (d == 0) error("divide by zero");
left /= d;
break;
}
default: {
ts.unget(t);
return left;
}
}
}
}

double expression()
{
double left = term();
while (true) {
Token t = ts.get();
switch (t.kind) {
case '+':
{
left += term();
break;
}
case '-':
{
left -= term();
break;
}
default:
{
ts.unget(t);
return left;
}
}
}
}

double declaration()
{
Token t = ts.get();
if (t.kind != name) { //if (t.kind != 'a')
error("name expected in declaration");
}

string name = t.name;
if (is_declared(name)) {
error(name, " declared twice");
}

Token t2 = ts.get();
if (t2.kind != '=') {
error("= missing in declaration of ", name);
}
double d = expression();
names.push_back(Variable(name, d));
return d;
}

double statement()
{
Token t = ts.get();
switch (t.kind) {
case let:
{
return declaration();
}
default:
{
ts.unget(t);
return expression();
}
}
}

void clean_up_mess()
{
ts.ignore(print);
}

const string prompt = "> ";
const string result = "= ";

void calculate()
{
while (true) try {
cout << prompt;
Token t = ts.get();
while (t.kind == print) {
t = ts.get();
}
if (t.kind == exitprog) {
return;
}
ts.unget(t);
cout << result << statement() << endl;
}
catch (runtime_error& e) {
cerr << e.what() << endl;
clean_up_mess();
}
}

int main()

try {
calculate();
return 0;
}
catch (exception& e) {
cerr << "exception: " << e.what() << endl;
char c;
while (cin >> c&& c != ';');
return 1;
}
catch (...) {
cerr << "exception\n";
char c;
while (cin >> c && c != ';');
return 2;
}
```
```Output:
> # x_4 = 5;
= 5
> x_4 + 6;
= 11
>
```

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