How to switch istream from cin to something else? - c++

Question

So I have been solving problems from this book called
Programming: Principles and Practice Using C++ by Bjarne Stroustrup to learn C++ and have been enjoying it a lot. I recently got stuck on this question in Chapter 10 (Input and Output
Streams).
The question is given below:
Add a command from x to the calculator from Chapter 7 that makes it
take input from a file x. Add a command to y to the calculator that
makes it write its output (both standard output and error output) to
file y.
The source code for the above mentioned calculator is given below:
/*
Simple Calculator
This program implements a basic expression calculator.
Input from cin; output to cout.
The grammar for input is:
Calculation:
Statement
Print
Quit
Statement:
Declaration
Expression
Declaration:
"let" Name "=" Expression
Print:
;
Quit:
q
Expression:
Term
Term '+' Expression //addition
Term '-' Expression //subtraction
Term:
Primary
Term '*' Primary //multiplication
Term '/' Primary //division
Term '%' Primary //mod division
Primary:
Number
'(' Expression ')'
'-' Primary
'+' Primary
Number:
floating-point-literal
Input comes from cin through the Token_stream called ts.
*/
#include "../../std_lib_facilities.h"
class Token //a very simple user-defined type
{
public:
char kind = ' '; //what kind of token
double value = 0; //for numbers: a value
string name = " "; //for strings: a name
Token(char ch) :kind{ ch } { } //initialize kind with ch
Token(char ch, double val) :kind{ ch }, value{ val } {} //initializes kind and value
Token(char ch, string n) :kind{ ch }, name{ n } { } //initializes kind and name
};
class Token_stream
{
public:
//user interface
Token get(); //get a Token
void putback(Token t); //put a Token back
void ignore(char c); //discard characters up to and including a c
private:
//implementation details
//(not directly accessible to users of Token_stream)
bool full{ false }; //is there a Token in the buffer?
Token buffer = 0; //here is where we keep a Token put back with putback()
};
class Variable
{
public:
string name = " ";
string const_name = " ";
double value = 0;
};
vector<Variable>var_table;
double get_value(string s)
//return the value of the Variable named s
{
for (const Variable& v : var_table) {
if (v.name == s) return v.value;
if (v.const_name == s) return v.value;
}
error("get: undefined variable ", s);
return 1;
}
void set_value(string s, double d)
//set the Variable named s to d
{
for (Variable& v : var_table)
{
if (v.name == s)
{
v.value = d;
return;
}
if (v.const_name == s)
{
error("set: cannot assign a new value to an existing constant");
return;
}
}
error("set: undefined variable", s);
}
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 constant = 'c'; //constant token
const string constdeclkey = "const"; //constant keyword
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
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
}
void Token_stream::ignore(char c)
//c represents the kind of 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 Token_stream::get()
{
if (full)
{
full = false; //do we already have a Token ready?
return buffer; //remove Token from buffer
}
char ch;
cin >> ch; //note that >> skips whitespace(space, newline, tab, etc)
switch (ch)
{
case ';': //for "print"
case 'q': //for "quit"
case '(':
case ')':
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{ number, val }; //let '8' represent a "a number"
}
default:
if (isalpha(ch))
{
string s;
s += ch;
while (cin.get(ch) && (isalpha(ch) || isdigit(ch) || ch == '_')) s += ch;
cin.putback(ch);
if (s == declkey) return Token(let); //declaration keyword
else if (s == constdeclkey) return Token(constant);
return Token{ name, s };
}
error("Bad Token");
return 1;
}
}
bool is_declared(string var)
//is var already in var_table?
{
for (const Variable& v : var_table)
{
if (v.name == var) return true;
}
for (const Variable& v : var_table)
{
if (v.const_name == var) return true;
}
return false;
}
double define_name(string var, double val)
//add (var, val) to var_table
{
if (is_declared(var)) set_value(var, val);
Variable v;
v.name = var;
v.value = val;
var_table.push_back(v);
return val;
}
double define_const_name(const string constant, double val)
//add (constant, val) to var_table
{
if (is_declared(constant)) error(constant, " constant declared twice");
Variable v;
v.const_name = constant;
v.value = val;
var_table.push_back(v);
return val;
}
Token_stream ts; //makes a Token_stream called ts that reads from cin
double expression(); //declaration so that primary() can call it
double declaration()
//assume we have seen "let"
//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 const_declaration()
//assume we have seen "const"
//handle: const_name = expression
//declare a constant called "const" with the initial value "expression"
{
Token t = ts.get();
if (t.kind != name) error("constant_name expected in declaration");
const string const_name = t.name;
Token t2 = ts.get();
if (t2.kind != '=') error("= missing in declaration of ", const_name);
double d = expression();
define_const_name(const_name, d);
return d;
}
double primary()
//deals with numbers and parantheses
//calls expression() and get_token()
{
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: //we use '8' to represent a number
return t.value; //return the number's value
case '-':
return -primary();
case '+':
return primary();
case name:
return get_value(t.name);
default:
error("primary expected");
return 1;
}
}
double term()
//deals with * and -
//calls primary and get_token()
{
double left = primary(); //read and evaluate a Primary
Token t = ts.get(); //get the next token
while (true)
{
switch (t.kind) //see which kind of token it is
{
case '*':
left *= primary(); //evaluate Primary and multiply
t = ts.get();
break;
case '/':
{
double d = primary(); //evaluate Primary and divide
if (d == 0) error("divide by zero"); //check if primary isnt zero
left /= d;
t = ts.get();
break;
}
case '%':
{
double d = primary();
if (d == 0) error("divide by zero");
left = fmod(left, d);
t = ts.get();
break;
}
default:
ts.putback(t); //put t back into Token stream
return left; //return result to Expression
}
}
}
double expression()
//deals with + and -
//calls term() and get_token()
{
double left = term(); //read and evaluate an Term
Token t = ts.get(); //get the next token from the Token stream
while (true)
{
switch (t.kind) //see which kind of token it is
{
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; //return the answer
}
}
}
void clean_up_mess() //B)
{
//skip until we find a print
ts.ignore(print);
}
double statement()
{
Token t = ts.get();
switch (t.kind)
{
case let:
return declaration();
case constant:
return const_declaration();
default:
ts.putback(t);
return expression();
}
}
void calculate() //expression evaluation loop
{
double val = 0;
while (cin)
try
{
cout << prompt; //print prompt
Token t = ts.get();
while (t.kind == print) //';' for "print now"
t = ts.get(); //eat ';'
if (t.kind == quit) //'q' for "quit"
{
return;
}
ts.putback(t);
cout << result << statement() << '\n';
}
catch (exception& e)
{
cerr << e.what() << '\n'; //write error message
clean_up_mess();
}
}
int main() //main loop and deals with errors
{
try
{
//predefined names:
define_const_name("pi", 3.1415926535);
define_const_name("e", 2.7182818284);
calculate();
keep_window_open();
return 0;
}
catch (exception& e)
{
cerr << e.what() << '\n';
keep_window_open("~~");
return 1;
}
catch (...)
{
cerr << "exception \n";
keep_window_open();
return 2;
}
}
The calculator kind of works like this:
> (2+3)-4*(5/6); //Input
= 1.66667 //Output
> //Next Input
It can also declare variables and constants and then use them back into for calculations:
> let x = 2; //Variable 1
= 2 //assigned value for Var 1
> const y = 3; //Constant 1
= 3 //assigned value Const 1
> x + y; //added var 1 to const 1
= 5 //result
> //next input
Also ; acts like a print character. The expression gets evaluated and displayed as soon as it reads ; and then it clears the token stream and gets ready to read the next input.
The custom header file std.lib.facilities.h is a collection of most standard C++ libraries and functions some of which are written by Bjarne Stroustrup and might have been used in the code given above.
I have also left the code for that header file just in case if someone is curious:
/*
std_lib_facilities.h
*/
/*
simple "Programming: Principles and Practice using C++ (second edition)" course header to
be used for the first few weeks.
It provides the most common standard headers (in the global namespace)
and minimal exception/error support.
Students: please don't try to understand the details of headers just yet.
All will be explained. This header is primarily used so that you don't have
to understand every concept all at once.
By Chapter 10, you don't need this file and after Chapter 21, you'll understand it
Revised April 25, 2010: simple_error() added
Revised November 25 2013: remove support for pre-C++11 compilers, use C++11: <chrono>
Revised November 28 2013: add a few container algorithms
Revised June 8 2014: added #ifndef to workaround Microsoft C++11 weakness
Revised Febrary 2 2015: randint() can now be seeded (see exercise 5.13).
Revised August 3, 2020: a cleanup removing support for ancient compilers
*/
#ifndef H112
#define H112 080315L
#include<iostream>
#include<iomanip>
#include<fstream>
#include<sstream>
#include<cmath>
#include<cstdlib>
#include<string>
#include<list>
#include <forward_list>
#include<vector>
#include<unordered_map>
#include<algorithm>
#include <array>
#include <regex>
#include<random>
#include<stdexcept>
//------------------------------------------------------------------------------
typedef long Unicode;
//------------------------------------------------------------------------------
using namespace std;
template<class T> string to_string(const T& t)
{
ostringstream os;
os << t;
return os.str();
}
struct Range_error : out_of_range { // enhanced vector range error reporting
int index;
Range_error(int i) :out_of_range("Range error: " + to_string(i)), index(i) { }
};
// trivially range-checked vector (no iterator checking):
template< class T> struct Vector : public std::vector<T> {
using size_type = typename std::vector<T>::size_type;
/* #ifdef _MSC_VER
// microsoft doesn't yet support C++11 inheriting constructors
Vector() { }
explicit Vector(size_type n) :std::vector<T>(n) {}
Vector(size_type n, const T& v) :std::vector<T>(n, v) {}
template <class I>
Vector(I first, I last) : std::vector<T>(first, last) {}
Vector(initializer_list<T> list) : std::vector<T>(list) {}
*/
using std::vector<T>::vector; // inheriting constructor
T& operator[](unsigned int i) // rather than return at(i);
{
if (i<0 || this->size() <= i) throw Range_error(i);
return std::vector<T>::operator[](i);
}
const T& operator[](unsigned int i) const
{
if (i<0 || this->size() <= i) throw Range_error(i);
return std::vector<T>::operator[](i);
}
};
// disgusting macro hack to get a range checked vector:
#define vector Vector
// trivially range-checked string (no iterator checking):
struct String : std::string {
using size_type = std::string::size_type;
// using string::string;
char& operator[](unsigned int i) // rather than return at(i);
{
if (i<0 || size() <= i) throw Range_error(i);
return std::string::operator[](i);
}
const char& operator[](unsigned int i) const
{
if (i<0 || size() <= i) throw Range_error(i);
return std::string::operator[](i);
}
};
namespace std {
template<> struct hash<String>
{
size_t operator()(const String& s) const
{
return hash<std::string>()(s);
}
};
} // of namespace std
struct Exit : runtime_error {
Exit() : runtime_error("Exit") {}
};
// error() simply disguises throws:
inline void error(const string& s)
{
throw runtime_error(s);
}
inline void error(const string& s, const string& s2)
{
error(s + s2);
}
inline void error(const string& s, int i)
{
ostringstream os;
os << s << ": " << i;
error(os.str());
}
template<class T> char* as_bytes(T& i) // needed for binary I/O
{
void* addr = &i; // get the address of the first byte
// of memory used to store the object
return static_cast<char*>(addr); // treat that memory as bytes
}
inline void keep_window_open()
{
cin.clear();
cout << "Please enter a character to exit\n";
char ch;
cin >> ch;
return;
}
inline void keep_window_open(string s)
{
if (s == "") return;
cin.clear();
cin.ignore(120, '\n');
for (;;) {
cout << "Please enter " << s << " to exit\n";
string ss;
while (cin >> ss && ss != s)
cout << "Please enter " << s << " to exit\n";
return;
}
}
// error function to be used (only) until error() is introduced in Chapter 5:
inline void simple_error(string s) // write ``error: s and exit program
{
cerr << "error: " << s << '\n';
keep_window_open(); // for some Windows environments
exit(1);
}
// make std::min() and std::max() accessible on systems with antisocial macros:
#undef min
#undef max
// run-time checked narrowing cast (type conversion). See ???.
template<class R, class A> R narrow_cast(const A& a)
{
R r = R(a);
if (A(r) != a) error(string("info loss"));
return r;
}
// random number generators. See 24.7.
inline default_random_engine& get_rand()
{
static default_random_engine ran; // note: not thread_local
return ran;
};
inline void seed_randint(int s) { get_rand().seed(s); }
inline int randint(int min, int max) { return uniform_int_distribution<>{min, max}(get_rand()); }
inline int randint(int max) { return randint(0, max); }
//inline double sqrt(int x) { return sqrt(double(x)); } // to match C++0x
// container algorithms. See 21.9. // C++ has better versions of this:
template<typename C>
using Value_type = typename C::value_type;
template<typename C>
using Iterator = typename C::iterator;
template<typename C>
// requires Container<C>()
void sort(C& c)
{
std::sort(c.begin(), c.end());
}
template<typename C, typename Pred>
// requires Container<C>() && Binary_Predicate<Value_type<C>>()
void sort(C& c, Pred p)
{
std::sort(c.begin(), c.end(), p);
}
template<typename C, typename Val>
// requires Container<C>() && Equality_comparable<C,Val>()
Iterator<C> find(C& c, Val v)
{
return std::find(c.begin(), c.end(), v);
}
template<typename C, typename Pred>
// requires Container<C>() && Predicate<Pred,Value_type<C>>()
Iterator<C> find_if(C& c, Pred p)
{
return std::find_if(c.begin(), c.end(), p);
}
#endif //H112
So to start with this problem I added the constant character from and constant string from_key just under all of the other constants in the calculator code.
const char from = 'f'; // file input read token
const string from_key = "from"; // file input read keyword
Then I added a new case for from token in the statement()'s function definiton
double statement()
{
Token t = ts.get();
switch (t.kind)
{
/.../
case from:
return input();
/.../
}
}
Ignore the function input() being returned from this case. That's kind of my main problem with this question. I will come back to it after the next part.
I followed it by adding a Token return for from's case in Token Token_stream::get() function's definition
Token Token_stream::get()
{
if (full)
{
/.../
}
char ch;
cin >> ch; //note that >> skips whitespace(space, newline, tab, etc)
switch (ch)
{
/.../
default:
if (isalpha(ch))
{
string s;
s += ch;
while (cin.get(ch) && (isalpha(ch) || isdigit(ch) || ch == '_')) s += ch;
cin.putback(ch);
if (s == declkey) return Token(let); //declaration keyword
else if (s == constdeclkey) return Token(constant);
else if (s == from_key) return Token(from);
return Token{ name, s };
}
error("Bad Token");
return 1;
}
}
After that I started writing the input() function which I mentioned seconds ago. It looks like this for now:
double input()
{
Token t = ts.get();
if (t.kind != name) error("read-file name expected in declaration");
string read_file = t.name;
ifstream ifs{ read_file + ".txt" };
if (!ifs) error("cannot open file " + read_file + ".txt");
while (ifs)
{
try
{
//idk what to do here
}
catch (exception& e)
{
cerr << e.what() << '\n';
clean_up_mess();
}
return 0;
}
}
So what I want to do is pretty simple.
I just want Token Token_stream::get() function to read input from ifs rather than cin, kind of like redirect the istream to ifs while its reading data from the file then switch it back to cin when its done. I'm thinking about calling the statement() function back here to print the evaluation on screen for now and then I might redirect it again to write the output to a file y.txt as it's mentioned in the question.
My issue is that I really don't know how to do this. I tried doing an if-else statement to switch istream from cin to ifs but getting ifs out from input()'s scope to Token Token_stream::get()'s scope is a big hurdle for me.