I am trying to learn cin.fail()

[lkordich]

I am trying to do input validation to make sure nothing but integers are entered but I cannot get it to work right. I have watched video tutorials. What is happening is the loop is not working correctly. If something other than an integer is input it goes to a blank screen and doesn't loop back and then I will put a number but it still does not proceed. This is the code

 #include <iostream>
#include <iomanip>
#include <cstdlib>
#include <limits>
using namespace std;

int main()
{
	int calories;
	bool bFail;
	do {
		cout << setw(5) << " " << "Enter calories: ";
		cin >> calories;

		bFail = cin.fail();
		cin.clear();
		cin.ignore(numeric_limits<streamsize>::max(), 'n');

	} while (bFail == true); 

    return 0;
}

[helios]

Did you mean to ignore '\n', rather than 'n'?

[lkordich]

Yes. Let me try that.

[lkordich]

That made it work. I also need to make it do validation in case 0 is entered for either fat grams or calories for the finished product. Is this possible to do another while loop to match that? Thank you for your help.

[helios]

Probably bFail = cin.fail() || calories == 0; would work. Or do you mean you need a second loop to receive input for fat grams, after calories?

[Manga]

How about bFail = cin.fail() || calories < 1;?

[Handy Andy]

Hello lkordich,

Here is another approach that I have seen many times and it has the advantage of catching more than just the fail bit.

do
{
	cout << setw(5) << " " << "Enter calories: ";
	cin >> calories;

	while (!cin)
	{
		std::cout << "\n Error message" << std::endl;
		std::cin.clear();
		std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');  // <--- Requires header file <limits>.

		cout << setw(5) << " " << "Enter calories: ";
		cin >> calories;

	}
} while (condition);

This tends to work best when the input is to a variable that is defined as a numeric variable.

Your approach will work, but keep in mind there is more than one state bit that could cause "cin" to fail.

Hope that helps,

Andy

[lkordich]

Thank you Andy. I got it solved but am still learning. I did use this way.