What is the correct way to use cin.fail();?
I am making a program where you need to input something. It is not very clear if you need to input a number or character. When a user inputs a character instead of a number the console goes crazy. How can I use cin.fail() to fix this?
Or is there a better way?
2 Answers
std::cin.fail() is used to test whether the preceding input
succeeded. It is, however, more idiomatic to just use the
stream as if it were a boolean:
if ( std::cin ) {
// last input succeeded, i.e. !std::cin.fail()
}
if ( !std::cin ) {
// last input failed, i.e. std::cin.fail()
}
In contexts where the syntax of the input permit either a number
of a character, the usual solution is to read it by lines (or in
some other string form), and parse it; when you detect that
there is a number, you can use an std::istringstream to
convert it, or any number of other alternatives (strtol, or
std::stoi if you have C++11).
It is, however, possible to extract the data directly from the stream:
bool isNumeric;
std::string stringValue;
double numericValue;
if ( std::cin >> numericValue ) {
isNumeric = true;
} else {
isNumeric = false;
std::cin.clear();
if ( !(std::cin >> stringValue) ) {
// Shouldn't get here.
}
}
cin.fail() returns true if the last cin command failed, and false otherwise.
An example:
int main() {
int i, j = 0;
while (1) {
i++;
cin >> j;
if (cin.fail()) return 0;
cout << "Integer " << i << ": " << j << endl;
}
}
Now suppose you have a text file - input.txt and it's contents are:
30 40 50 60 70 -100 Fred 99 88 77 66
When you will run above short program on that, it will result like:
Integer 1: 30
Integer 2: 40
Integer 3: 50
Integer 4: 60
Integer 5: 70
Integer 6: -100
it will not continue after 6th value as it quits after reading the seventh word, because that is not an integer: cin.fail() returns true.
