/**
 * Example of look-ahead for input streams.
 */
 
#include <iostream>
#include <fstream>
#include <sstream>
#include <string>

using namespace std;

/**
 * Checks the stream to determine the type of each character until a newline
 * or EOF.
 */
void checkStream(istream &in)
{
	bool stop=false;
	while(!in.eof() && !stop)
	{
		switch(in.peek())
		{
			// Ignore whitespace
			case ' ': 
			case '\t': 
			case '\r': 
				in.get(); // Actually remove character from stream
				break;
			
			// Set stop flag when newline is encountered
			case '\n': 
				stop=true;
				in.get();
				break;
			
			// Digits
			case '0':
			case '1':
			case '2':
			case '3':
			case '4':
			case '5':
			case '6':
			case '7':
			case '8':
			case '9':
				// peek() and get() return an int so you will need to 
				// cast to a char if you want the symbol and not the 
				// ASCII code printed
				cout<<static_cast<char>(in.get())<<" digit"<<endl;
				break;
				
			// Operators
			case '-':
			case '+':
			case '*':
			case '/':
			case '%':
			case '^':
				cout<<static_cast<char>(in.get())<<" operator"<<endl;
				break;
				
			// Parenthesis
			case '(':
			case ')':
				cout<<static_cast<char>(in.get())<<" parenthesis"<<endl;
				break;
				
			// This means the end of the stream has been reached
			case -1: 
				in.get(); // This will trigger the EOF flag
				break;
		}
	}
}

int main()
{
	cout<<"Enter an expression: ";
	checkStream(cin);
	
	// Expression from strings
	string expr="2*3(-6  \t \t + 5   /(12  )\t)";
	cout<<endl<<"Processing from string: "<<expr<<endl;
	istringstream istr(expr);
	checkStream(istr);
	
	// Write expression to file
	ofstream out("temp_expr_file");
	if(out)
		out<<expr<<endl;
	else
	{
		cerr<<endl<<"Error: could not write file"<<endl;
		return 1;
	}
	out.close();
	
	// Read from file
	ifstream in("temp_expr_file");
	cout<<endl<<"Processing expression from file: "<<expr<<endl;
	if(in)
		checkStream(in);
	else
	{
		cerr<<endl<<"Error: could not read file"<<endl;
		return 2;
	}
	in.close();
}