I am trying to write a code to prompt user input from cin.
int main()
{
int year;
cout << "Enter a valid year: ";
cin >> year;
while (cin.fail())
{
cout << "Re-enter a valid value! ";
}
}
However, the compiler keeps printing the line "Re-enter a valid value! " unterminatedly when I enter a non-integer type. I don't know what's wrong about my code! Can somebody please correct it? Thank you very much.
Try this:
bool done = false;
int year;
for (std::string line;
std::cout << "Enter a year: " && std::getline(std::cin, line); )
{
std::istringstream iss(line);
if (iss >> year >> std::ws && iss.get() == EOF) { done = true; break; }
std::cerr << "Failed to parse input '" << line << "', please try again.\n";
}
if (!done) { std::cerr << "Premature end of input.\n"; }
else { std::cout << "Input: " << year << "\n"; }
istream::clear() and istream::ignore() approach without an extra stream?'" << line << "' un-sanitized I/O? Maybe I've been writing web apps and worrying about cross site scripting too long?while (cin.fail()) is not working.
This is my approach :
int main() {
int year;
while (true) {
cout << "Enter a valid year: ";
cin >> year;
if (cin.fail()) {
cin.clear(); cin.ignore();
cout << "Re-enter a valid value! " << endl;
} else break;
}
return 0;
}
if(cin) or if(!cin) somehow being superior to ios::good(), ios::bad(), and ios::fail() but it could be cin.fail() == !cin in all cases and I just forget that all the time.
xyz1234abc as valid input. I don't know whether that behaviour is preferable over the alternative, but it's worth calling out the difference.The problem you've described is here:
while (cin.fail())
{
cout << "Re-enter a valid value! ";
}
It looks like you don't fully understand what's happening here yet, so I will break this down for you.
The code translates to something like this:
"While cin.fail() is true, output "Re-enter a valid value! " to the console"
Which it will do continuously because there is no way to break out of the loop.
I don't think cin.fail() is what you want to use here, it looks like you want an algorithm to tell you if the date is valid or not and, if it's not valid, to repeat the query to the user so they can enter good data. Conceptually this is good practice, you're on the right track, but you need to learn a bit more.
I would suggest reading some tutorials on input/output and also looking into input validation.
This is an excellent resource for learning C++ http://www.cplusplus.com/doc/tutorial/
And here is some info on input validation http://www.cplusplus.com/forum/beginner/121194/
cin, you also need to reset the error state on the stream object.