1. ## A simple calculator

Hello guys,i have an assigment which includes an exercise about creating a calculator gui(fltk).This is the algorithm used from professor Stroustrup without the gui.
Code:
```//
// This is example code from Chapter 7.8.3 "Predefined names" of
// "Programming -- Principles and Practice Using C++" by Bjarne Stroustrup
//

#include "std_lib_facilities.h"

/*
Simple calculator

Revision history:

Revised by Bjarne Stroustrup May 2007
Revised by Bjarne Stroustrup August 2006
Revised by Bjarne Stroustrup August 2004
Originally written by Bjarne Stroustrup
(bs@cs.tamu.edu) Spring 2004.

This program implements a basic expression calculator.
Input from cin; output to cout.

The grammar for input is:

Calculation:
Statement
Print
Quit
Calculation Statement

Statement:
Declaration
Expression

Declaration:
"let" Name "=" Expression

Print:
;

Quit:
q

Expression:
Term
Expression + Term
Expression - Term
Term:
Primary
Term * Primary
Term / Primary
Term % Primary
Primary:
Number
Name
( Expression )
- Primary
+ Primary
Number:
floating-point-literal

Input comes from cin through the Token_stream called ts.
*/

//------------------------------------------------------------------------------

const char number = '8';    // t.kind==number means that t is a number Token
const char quit   = 'q';    // t.kind==quit means that t is a quit Token
const char print  = ';';    // t.kind==print means that t is a print Token
const char name   = 'a';    // name token
const char let    = 'L';    // declaration token
const string declkey = "let";// declaration keyword
const string prompt  = "> ";
const string result  = "= "; // used to indicate that what follows is a result

//------------------------------------------------------------------------------

class Token {
public:
char kind;        // what kind of token
double value;     // for numbers: a value
string name;      // for names: name itself
Token(char ch)             : kind(ch), value(0)   {}
Token(char ch, double val) : kind(ch), value(val) {}
Token(char ch, string n)   : kind(ch), name(n)    {}
};

//------------------------------------------------------------------------------

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
void ignore(char c);      // discard tokens up to an including a c
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() // read characters from cin and compose a Token
{
if (full) {         // check if we already have a Token ready
full=false;
return buffer;
}

char ch;
cin >> ch;          // note that >> skips whitespace (space, newline, tab, etc.)

switch (ch) {
case quit:
case print:
case '(':
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':    // numeric literal
{
cin.putback(ch);// put digit back into the input stream
double val;
cin >> val;     // read a floating-point number
}
default:
if (isalpha(ch)) {
string s;
s += ch;
while (cin.get(ch) && (isalpha(ch) || isdigit(ch))) s+=ch;
cin.putback(ch);
}
}
}

//------------------------------------------------------------------------------

void Token_stream::ignore(char c)
// c represents the kind of a Token
{
// first look in buffer:
if (full && c==buffer.kind) {
full = false;
return;
}
full = false;

// now search input:
char ch = 0;
while (cin>>ch)
if (ch==c) return;
}

//------------------------------------------------------------------------------

Token_stream ts;        // provides get() and putback()

//------------------------------------------------------------------------------

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

//------------------------------------------------------------------------------

vector<Variable> var_table;

//------------------------------------------------------------------------------

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

//------------------------------------------------------------------------------

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

//------------------------------------------------------------------------------

bool is_declared(string var)
// is var already in var_table?
{
for (int i = 0; i<var_table.size(); ++i)
if (var_table[i].name == var) return true;
return false;
}

//------------------------------------------------------------------------------

double define_name(string var, double val)
{
if (is_declared(var)) error(var," declared twice");
var_table.push_back(Variable(var,val));
return val;
}

//------------------------------------------------------------------------------

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 number:
return t.value;    // return the number's value
case name:
return get_value(t.name); // return the variable's value
case '-':
return - primary();
case '+':
return primary();
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;
}
case '%':
{
int i1 = narrow_cast<int>(left);
int i2 = narrow_cast<int>(term());
if (i2 == 0) error("%: divide by zero");
left = i1%i2;
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
}
}
}

//------------------------------------------------------------------------------

double declaration()
// handle: name = expression
// declare a variable called "name" with the initial value "expression"
{
Token t = ts.get();
if (t.kind != name) error ("name expected in declaration");
string var_name = t.name;

Token t2 = ts.get();
if (t2.kind != '=') error("= missing in declaration of ", var_name);

double d = expression();
define_name(var_name,d);
return d;
}

//------------------------------------------------------------------------------

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

//------------------------------------------------------------------------------

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

//------------------------------------------------------------------------------

void calculate()
{
while (cin)
try {
cout << prompt;
Token t = ts.get();
while (t.kind == print) t=ts.get();    // first discard all "prints"
if (t.kind == quit) return;        // quit
ts.putback(t);
cout << result << statement() << endl;
}
catch (exception& e) {
cerr << e.what() << endl;        // write error message
clean_up_mess();
}
}

//------------------------------------------------------------------------------

int main()
try {
// predefine names:
define_name("pi",3.1415926535);
define_name("e",2.7182818284);

calculate();

keep_window_open();    // cope with Windows console mode
return 0;
}
catch (exception& e) {
cerr << e.what() << endl;
keep_window_open("~~");
return 1;
}
catch (...) {
cerr << "exception \n";
keep_window_open("~~");
return 2;
}

//------------------------------------------------------------------------------```
If you notice,in Token Token_stream::get(),there is use of a char called ch to input data.

The aim of the exercise is to input in a box the things you want to calcualte and than you will output the result in another box.

Please note i already tried breaking the string into an array of characters and didnt do the job.

tl;dr I need a way to stream a string,character by character.