cin fails to stop while loop

Question

I was wondering if you guys could explain to me why this loop doesn't loop if I enter 'r' in for the value of 'value'.

double value

std::cout << "Please enter a real number: ";
std::cin  >> value;

while (!isdigit(value))
{
    std::cout << "Sorry, but only numbers are valid.\nPlease enter a real number: ";
    std::cin  >> value;
}

Thank you very much.

isdigit checks if the passing value is a number and nothing else. Search for it on cplusplus — Haakon Sjøgren, Feb 17 '14 at 23:16
If you enter a non-digit character for `value`, it is considered a failed input operation. The character is *not* extracted into `value`. — David G, Feb 17 '14 at 23:17
_'value is a number'_ you missed reading further _value is a number **character**_ — πάντα ῥεῖ, Feb 17 '14 at 23:18
Oops, I just saw your edit that value is a double. See this for how to do what you want: http://stackoverflow.com/questions/514420/how-to-validate-numeric-input-c isdigit is probably not what you want. — m24p, Feb 17 '14 at 23:19

Eugene S · Answer 1 · 2014-02-17T23:22:38.667

2

You shouldn't be using isdigit() here because that's for testing whether a single char type is an ASCII digit.

If you're expecting the user to enter a real number (and not a string), then you should use cin.fail() as your loop test condition. If cin reads a string when it's expecting a double then cin.fail() will return true.

Another (cleaner) option is to just test the state of cin as @bennofs pointed out:

while (!(std::cin >> value))
{
    // bad value was entered
}

edited Feb 17 '14 at 23:22

answered Feb 17 '14 at 23:17

Eugene S

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or just use the result of `std::cin >> value` – bennofs Feb 17 '14 at 23:18

David Hammen · Answer 2 · 2014-02-17T23:23:59.063

1

You did two things wrong: You declared value as a double, and you used std::cin >> value to populate that incorrectly declared variable.

You need to read a character, so read a character. Declare value as a char.

Edit: I am assuming your assignment is to read a character and see if it represents one of the numeric digits. If your assignment is to read a number, you are going about this all wrong.

edited Feb 17 '14 at 23:23

answered Feb 17 '14 at 23:17

David Hammen

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cin fails to stop while loop

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