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
            ([email protected]) 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 '=':
        return Token(ch); // let each character represent itself
    case '.':            // a floating-point literal can start with a dot
    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
        return Token(number,val);
    }
    default:
        if (isalpha(ch)) {
            string s;
            s += ch;
            while (cin.get(ch) && (isalpha(ch) || isdigit(ch))) s+=ch;
            cin.putback(ch);
            if (s == declkey) return Token(let); // keyword "let"
            return Token(name,s);
        }
        error("Bad token");
    }
}

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

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)
    // add (var,val) to var_table
{
    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;
}

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

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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.

Thanks in advance.

You can enumerate all characters in a string:

for (auto elem : str)
// elem is your character

// Or for pre-C++11
for (std::string::iterator it = str.begin(); it != str.end(); ++it)
// *it is your character