C ++如何让用户输入方程式求值

Question

My program is meant to ask a user to input an equation. Then find the maximum over an interval given by the user. When I compile my program, the output I get is:

Please complete the equation to be evaluated f(x)= 
Please enter the first number of the interval to be checked: 
Please enter the last number of the interval to be checked: 
Please enter the desired initial step size: 
sh: PAUSE: command not found

with the last line repeating many times. I think the problem here has something to do with having the user input the equation to be tested. However, I'm unsure of how to fix this.

Here's my code

#include <iostream>
#include <cmath>
#include <cstdlib>

using namespace std;

int main()
{
    int a, b, delta, fx, x, y;
    int max = 0;

    cout <<"Please complete the equation to be evaluated f(x)= " << endl;
    cin >> fx;
    cout <<"Please enter the first number of the interval to be checked: " << endl;
    cin >> a;
    cout << "Please enter the last number of the interval to be checked: " << endl;
    cin >> b;
    cout << "Please enter the desired initial step size: " << endl;
    cin >> delta;

    for(x = a; x <= b; x = x+delta)                                
    {
        y = fx;     
        if (y > max)  
        { 
            max = y;  
            cout <<"The maximum over the interval from " << a <<"to " << b <<"is " << delta;
        }
        else
        {
            delta= delta/2;
        }
        if (delta <  pow( 10, -6))
        {
            system ("PAUSE");
        }
    }      

    return 0;
}

Answer 1

Simply don't use system("pause"); in the if statement and you'll lose that error: "sh: PAUSE: command not found" . Place it right before the end of the main.

system("pause");
return 0;

Answer 2

F(x) shouldn't be an integer variable, it should be a string variable. That way, the user can enter operators as characters instead of the compiler thinking they should be numbers. You would then have to process the string to determine the equation; this would require some thought, and possibly a more advanced data structure such as a binary tree.

Answer 3

As pointed out by others, the form of f(x) could be an issue with the above code.

Consider to redesign what to achieve for your program. One possibility is to narrow down the f(x) as polynomial function so that you can avoid parsing general algebraic equation, in this case you can ask: how many degree of the polynomial ? upon this, it is followed by input the coefficient value for each factor in the polynomial equation.

This way, you can still use integer (or double - better) in the program.