CODEWITHSUNDEEP
c++loopswhile-loop
Haakon Sjøgren
Haakon Sjøgren

Reputation: 5

cin fails to stop while loop

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.

Upvotes: 0

Views: 232

Answers (2)

David Hammen
David Hammen

Reputation: 33106

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.

Upvotes: 1

Eugene S
Eugene S

Reputation: 3122

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
}

Upvotes: 2

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