C++ check input value with while loop

Question

I have the following problem. I try to check the format of the input value with the while loop. If the input is wrong, I want to get back and ask the user to give a new input. But this step is just skipped and it continues with the rest of the code. How can I fix it? Thanks in advance! P.S.: Credits is a double.

cout << "Enter amount of credits: "; 
cin >> credits;
while(cin.fail()){
    cout<<"Wrong input! Please enter your number again: ";
    cin>> credits;
}

`cin.fail()` doesn't verify the validity of the input. And to do that, we'd need first to establish what does "valid" mean. — Bartek Banachewicz, Apr 22 '16 at 09:41
http://en.cppreference.com/w/cpp/io/basic_ios/fail refer this — Undefined Behaviour, Apr 22 '16 at 09:41
I assume by validity of input, you want to check weather input has correct data type or not ? — Tejendra, Apr 22 '16 at 09:43
Possible duplicate: [C++ to check if user input is a number, not a character or a symbol](http://stackoverflow.com/questions/19018294/c-to-check-if-user-input-is-a-number-not-a-character-or-a-symbol) — Rowen Chumacera, Apr 22 '16 at 09:51

score 2 · Accepted Answer · edited May 23 '17 at 11:59

You can validate the data type of input provided in a very simple way

cout << "Enter amount of credits: "; 
while(!(cin>> credits)){
    cout<<"Wrong input! Please enter your number again: ";
    cin.clear();
    cin.ignore(numeric_limits<streamsize>::max(), '\n');
}

ignore is necessary to clear the standard input, as mentioned in reference below since operator>> won't extract any data from the stream anymore as it is in a wrong format

For more reference you can check

score 0 · Answer 2 · answered Apr 22 '16 at 10:08

Your original code works with this modification:

while (std::cin.fail())
{
    std::cout << "Wrong input! Please enter your number again: ";
    std::cin.clear();
    std::cin.ignore();
    std::cin >> credits;
}

As Tejendra mentioned, cin.clear and cin.ignore is the key.

C++ check input value with while loop

2 Answers2