Input handling

[leokz145]

I am trying to use just cin in a while loop to catch when the user inputs a non integer to the command line. Also could someone explain how cin is acting like a boolean for the condition in the loop. Here is my code:

        cout << "Enter coefficient a: ";
        cin >> coeffs[0];

        while(!cin){
                int i = 0;
                cout << "Please rememeber that you must enter a non-zero number for a. Try again." << endl << endl;
                cout << "Enter coefficient a: ";
                cin >> coeffs[0];
                if(i == 4){
                        cout << "Too many failed attempts the program exits." << endl <<endl;
                        exit(2);
                }
                i++;
        }

Edit: I think I understand now how cin is used for the condition statement, correct me if I am wrong. You declare a variable with a certain type and then take the input and if the input matches the type then it is true otherwise false.

[LB]

std::cin is a std::istream, which has a conversion operator to implicitly convert it to a bool.

You need to remember to clear the error state and ignore the bad input before trying for input again.

[leokz145]

Ah that makes sense, so this would be the correct way of doing it:

        cout << "Enter coefficient a: ";
        cin >> coeffs[0];

        while(!cin){
                int i = 0;
                cout << "Please rememeber that you must enter a non-zero number for a. Try again." << endl << endl;
                cin.clear();
                cin.ignore(80, '\n');

                cout << "Enter coefficient a: ";
                cin >> coeffs[0];
                if(i == 4){
                        cout << "Too many failed attempts the program exits." << endl <<endl;
                        exit(2);
                }
                i++;
        }

[LB]

80 is an arbitrary number, and the user could easily enter more than that. The correct way is to use the maximum value the stream supports:

std::cin.clear();
std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

You will need to #include <limits> .

Other than that, it seems like it should work - does it?

[leokz145]

It does for the most part the only thing is that I guess maybe variable "i" isn't updating therefore it never enters that if statement on line 12

[LB]

Look where you declare i.

[leokz145]

I tried declaring outside the loop and still got the same problem. Where should it be declared?

Edit: Nevermind declaring outside the loop worked, but why does it make a difference?

[LB]

When you declare it inside the loop, it gets redeclared every iteration of the loop, and thus reset to 0.