How to prevent an infinite loop, if a non-integer is entered?

Question

I'm writing a casino game on C++ turbo (latest version for windows). So in one particular snippet of the program, the user is required to enter an initial amount strictly between 0 dollars and 100000 dollars.

I created a do-while loop with an embedded if statement:

do{
    cout << "\n\nEnter a deposit amount (between $0 and $100000) to play game : $";
    cin >> amount;
    if(amount<=0||amount>=100000)
        cout<<"Please re-enter your amount";
}while(amount<=0||amount>=100000);

The issue arises when the user (i.e. me) enters a character or a decimal; the program then loses control and it keeps looping indefinitely.

Question: How can I phrase an if-statement which requests the user to re-enter the amount if something other than an integer is entered? How can I, subsequently, prevent the program from going out of control?

What don't you understand in particular? Keypoints are calling `clear()` and consuming the non numerical input. — πάντα ῥεῖ, Jan 03 '16 at 11:36
So in my case how do I consume the non-numerical input? There's nothing like .fail in TURBO C++... So I don't know what that means... @πάνταῥεῖ — Panglossian Oporopolist, Jan 03 '16 at 11:37
I know, the issue is my school requires me to do my project using this crappy compiler :( @πάνταῥεῖ — Panglossian Oporopolist, Jan 03 '16 at 11:40
There are many SO questions, like your question, in which the problem is that the error and/or remainder of line needs to be flushed after the user enters something incorrect. But at the moment I don't recall the exact syntax nor keywords to search for. — JSF, Jan 03 '16 at 11:48
@Kugelblitz Ha, my school doesn't care; as long as the code compiles, it is all OK. — Flare Cat, Jan 04 '16 at 20:55

score 3 · Answer 1 · answered Jan 03 '16 at 11:50

The problem is that when you call cin >> amount and the input is not numeric, the data is left in the buffer. Once your code loops back to the same operation, your read fails again, in an infinite loop.

To fix this problem you need to check the result of cin >> amount, like this:

if (cin >> amount) {
    ... // do something with amount
} else {
    cin.clear(); // unset failbit
    // Ignore some input before continuing
    cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
}

Reference on the ignore function is here.

score 2 · Answer 2 · edited May 23 '17 at 10:28

You have to clear the error flag and ignore the characters.

For an example of cin.clear() see here.

do {
    cout << "Enter a deposit amount (between $0 and $100000) to play game : $";
    cin >> amount;

    if(amount<=0 || amount>=100000) {
        cout << "\nPlease re-enter your amount\n";
        cin.clear();
        cin.ignore(10000,'\n');
    }
} while(amount<=0 || amount>=100000);

Support Ukraine · Accepted Answer · 2016-01-03T12:01:30.417

1

Try this

    if(amount<=0||amount>=100000)
    {
        cout<<"Please re-enter your amount";
        cin.clear();
        cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
    }

edited Jan 03 '16 at 12:01

answered Jan 03 '16 at 11:51

Support Ukraine

42,271
4
38
63

Fantastic. +1 my friend... I don't get the cin.ignore part... why is there a 10000? – Panglossian Oporopolist Jan 03 '16 at 11:57
Nvm... I got to know. But thank you! – Panglossian Oporopolist Jan 03 '16 at 11:59
2

Notice: I think the `10000`is just a bad habbit. It probably better to do `std::numeric_limits::max()` – Support Ukraine Jan 03 '16 at 12:00
Yea I know,,, but it's alright. – Panglossian Oporopolist Jan 03 '16 at 12:14

How to prevent an infinite loop, if a non-integer is entered?

3 Answers3