pls help: postfix expression evaluation by reading data from text file

Hi All
pls i need urgent help i have written a code that takes input from users and then evaluate the postfix expression but i don't know how to do it if data and even expressions are to be read from a text file...

Code:

#include <iostream>
#include <stack> 
#include <string>
using namespace std;
#include <fstream>
using std::ifstream;
#include <cstdlib>

int main()
{
    int i, choice = 1;
    string postfixExp;
    char token;
    float value, value1, value2;
    stack<float> s; //Declare a stack of floats
    
	/* i only know how to open my file and read data from it but don't know if i need to read it in certain format*/
	ifstream indata;
	indata.open("C:\\temp\\575_prog2_data.txt");
	if(!indata)  {
		cerr << "Error: File could not be opened" << endl;
		exit (1);
	}

	char letter;
	while (indata.eof()==0) 
	{indata >> letter;
	cout <<letter;
	cout << endl;
	indata.close();
	cout<< "end of file reached.." <<endl;
	return 0;
	}



    while (choice != 0)
    {
        cout << "1. Evaluate a postfix expression" << endl;
        cout << "0. Exit " << endl;
        cout << "Enter the number for the option: ";

        cin >> choice;
        switch(choice)
        {
            case 1: cout << "Evaluate a postfix expression\n";
                    cout << "Enter the expression: ";
                    cin >> postfixExp;
                    i = 0;
                    token = postfixExp[i];
                    while((i < postfixExp.size()) && (token != '='))
                    {
                        if(isdigit(token))
                        {
                            value = token - '0';
                            s.push(value);
                        }
                        else
                        {
                            value2 = s.top();
                            s.pop();
                            value1 = s.top();
                            s.pop();
                            switch(token)
                            {
                                case '+': value = value1 + value2;
                                          break;
                                case '-': value = value1 - value2;
                                          break;
                                case '*': value = value1*value2;
                                          break;
                                case '/': value = value1/value2;
                                          break;
                            }
                            s.push(value);
                        }
               i++;
               token = postfixExp[i];
                    }
                    value = s.top();
                    s.pop();
                    cout << postfixExp << " " << value << endl;         
                    break;

            case 0: cout << "Exiting the program\n";
                    break;

            default: cout << "Invalid option\n";
                    break;
        }
     cout << endl;
    }
}