Question

I’m learning C++.

nav is an integer .

I want to ask user for typing a valid value, if he / she type an invalid value.

void main()
{
    printf("Type an integer : ");
    if(!scanf("%d", &nav))
    {
        system("cls");
        printf("Invalid ! \n");
        main();
    }
}

But it’s blinking after typing first value . It’s blinking like reloading screen. I think it’s infinite loop.

How can i do it in right way ? I want to ask a number from users, until it’s typing a real number .

Answer 1

If the user types an invalid input, scanf() won’t consume it, and you’ll be left peeking the same offending input character forever. You need to first read whatever the user enters — I recommend using std::getline() — and then try to parse that with strtol(), sscanf() or std::istringstream. Don’t use atoi() because it doesn’t report failures.

int nav;
{
    string line;
    while (getline(cin, line))
        if (istringstream(line) >> nav)
            break;
}

EDIT: See the comments for a rather beautiful rendition of the above logic. I’ve left it out of the answer because: a) I don’t want to steal someone else’s idea, and b) I’m not sure I’d present a newcomer to C++ with that formulation — not in one go, at least.

P.S.: You can’t call main() in C++.