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
– Bartek Banachewicz, Commented Apr 22, 2016 at 9:41
I assume by validity of input, you want to check weather input has correct data type or not ? — Tejendra
– Tejendra, Commented Apr 22, 2016 at 9:43
Possible duplicate: C++ to check if user input is a number, not a character or a symbol — Rowen Chumacera
– Rowen Chumacera, Commented Apr 22, 2016 at 9:51

Community · Accepted Answer · 2017-05-23 11:59:52Z

2

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

edited May 23, 2017 at 11:59

CommunityBot

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answered Apr 22, 2016 at 9:47

Tejendra

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Comments

David · Accepted Answer · 2016-04-22 10:08:28Z

0

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.

answered Apr 22, 2016 at 10:08

David

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