Evaluating expression statements from a text file in C++ - c++

Question

I am altering a (well, my first) C++ program that evaluates expressions. For example, by inputting this:
(x + (y * 3)), x = 2, y = 6;
It should output the following:
Value = 20
The program works when the user types the expression into the console, but I want it to read and evaluate expressions from a text file (each on a separate line). I have successfully read the file in, and can even print its contents in the console, but I'm having trouble parsing and evaluating each line. There are many more header and source files that accompany this project, but I think all of the relevant information is in the main function:
#include <iostream>
#include <string>
#include <fstream>
using namespace std;
#include "expression.h"
#include "subexpression.h"
#include "symboltable.h"
#include "parse.h"
SymbolTable symbolTable;
void parseAssignments();
int main()
{
Expression* expression;
char paren, comma;
string program;
ifstream myfile("expressions.txt");
if (myfile.is_open())
{
while (getline(myfile, program))
{
cout << program << '\n'; //confirms that file was read properly
//code to parse expressions and print results
}
myfile.close();
}
else cout << "Unable to open file";
//original code that parses expression the user inputs
/*cout << "Enter expression: ";
cin >> paren;
expression = SubExpression::parse();
cin >> comma;
parseAssignments();
cout << "Value = " << expression->evaluate() << end1;*/
cin.get(); //added this so the window doesn't close automatically
return 0;
}
I have tried numerous approaches over the last couple days, and I think it comes down to my C++ ignorance of what exactly is happening on these two lines. My theory is that it parses everything to the right of the parenthesis in the first line, and everything to the right of the comma in the second line (ignoring the fact that the scopes would overlap).
cin >> paren;
cin >> comma;
Therefore, among other approaches, I tried including the following in the while loop. I am using MS Visual Studio, and when I click "> Local Windows Debugger" it will open up a console window with the first expression printed and a blinking line like it's waiting for me to input something. I close out of the window, and instead of a "> Continue" in place of "> Local Windows Debugger" it will just end the program. It is entirely possible that I'm just using Visual Studio incorrectly.
myfile >> paren;
expression = SubExpression::parse();
myfile >> comma;
parseAssignments();
cout << "Value = " << expression->evaluate();
And this:
std::istringstream iss(program);
iss >> paren;
expression = SubExpression::parse();
iss >> comma;
parseAssignments();
cout << "Value = " << expression->evaluate();
And of course this:
getline(myfile, program) >> paren;
expression = SubExpression::parse();
getline(myfile, program) >> comma;
parseAssignments();
cout << "Value = " << expression->evaluate();
Am I taking the completely wrong approach here, or worse should I be focusing on changing other functions as well? Thank you!!
EDIT: It appears that cin >> was being called in parse() and a few other .cpp files, which prevented my three attempts above from having much of an effect. Right now I'm looking into including the line as an argument in functions like below. I will report any errors if it doesn't work.
string parseName()
{
char alnum;
string name = "";
cin >> ws;
while (isalnum(cin.peek()))
{
cin >> alnum;
name += alnum;
}
return name;
}
EDIT 2: IT WORKS!!! Since cin >> was being used throughout the program, I had to create a new variable std::stringstream in(program); and then use that as arguments in functions as seen below:
void parseAssignments(stringstream& in);
int main()
{
Expression* expression;
char paren, comma;
string program;
ifstream myfile("expressions.txt");
if (myfile.is_open())
{
while (getline(myfile, program))
{
cout << program << '\n'; //confirms that file was read properly
std::stringstream in(program);
in >> paren;
expression = SubExpression::parse(in);
in >> comma;
parseAssignments(in);
cout << "Value = " << expression->evaluate() << '\n\;
}
myfile.close();
}
else cout << "Unable to open file";
cin.get(); //added this so the window doesn't close automatically
return 0;
}
Thanks for your help guys! I think I'm going to go back to something a bit simpler with C++ and build up from there...

Answer 1

Lets start with this
cin >> paren;
expression = SubExpression::parse();
cin >> comma;
This code reads a single character from cin (the terminal), then calls parse, then reads another single character. It never reads your expression. It also never does anything with the two single characters, it just throws them away.
This says to me that the code that reads the expression from the terminal is in SubExpression::parse() -- that code (which you don't show) apparently reads directly from cin.
This indicates immediately why all your attempts have failed -- you have your expression in a file or a string, but your parse routine is still stubbornly reading from cin. You need to change that routine to read from somewhere else.