How can I get my program to ignore character inputs?

[MobyNinja]

I am trying to set up a menu for which you are presented with 2 options (1 or 2). If a different number is typed, I got it so that the program rejects it. However, if an alpha character is typed, the program just wigs out. How can I get it to ignore alpha characters and only accept numbers?

[hamsterman]

This is a common question so I'll just link to an older post of mine.
http://www.cplusplus.com/forum/beginner/28223/#msg151683
Though there might be cleaner ways to write it..
Anyway, the idea is to evaluate cin>>to a boolean and cin.clear and retry if it's a 0. Alternatively you could use cin.fail to find an error.

[Maese909]

I wrote this code to test OP's question and for some reason when a character other than a number is entered, it causes an infinite loop. I am not sure why though...

int main()
{
	int num;

	cout << "Please enter 1 or 2: ";
	cin >> num;

	while(!cin || (num<=0 || num>=3))
	{
	    cout << "Invalid entry. Number must be 1 or 2. Please reenter number: ";
	    cin >> num;
	}

	return 0;
} 

[MobyNinja]

Thanks, that helped!

Unforunately, there are lots of places in my program that require the input of numbers, and I dont want to copy that whole code all over my program. I'm sure I can figure out an easier way of doing it if I sat here and thought about it...

[Sputnik]

The best option is to do it in one statement!

while ((c=getchar())==isalpha(c))
;

[ModShop]

Perhaps you should make an input function rather than using cin directly in your code.

int getInt(int min, int max);

Then input would be:

int num = getInt(1, 2);

Rather than:

while(!cin || (num<=0 || num>=3))
{
    cout << "Invalid entry. Number must be 1 or 2. Please reenter number: ";
    cin >> num;
}

The body of getInt() would be pretty similar to the latter.

[Duthomhas]

How about making a function?

#include <iostream>
#include <sstream>
#include <string> 

template <typename Result>
Result get_input(
  Result             default_value,
  Result             minimum_value,
  Result             maximum_value,
  const std::string& start_message,
  const std::string& try_again_message,
  const std::string& bad_range_message
  ) {
  Result result( default_value );

  std::cout << start_message;

  while (true)
    {
    std::string s;
    std::getline( std::cin, s );

    s.erase( s.find_last_not_of( " \f\n\r\t\v" ) + 1 );
    if (s.empty()) break;

    std::istringstream ss( s );
    ss >> result;

    if (!ss.eof())
      std::cout << try_again_message;

    else if ((minimum_value > result) or (result > maximum_value))
      std::cout << bad_range_message;

    else break;
    }

  return result;
  }

Use it thusly:

int x = get_input(
  50,
  1,
  100,
  "I am thinking of a number between 1 and 100, inclusive.\nWhat is it? ",
  "That was not a number. Try again: ",
  "The number must be between 1 and 100, inclusive: "
  );
cout << "Good job! You guessed the number " << x << ".\n";

Hope this helps.

[MobyNinja]

I did what Duoas sugested in the thread that hamsterman linked me to:

int main()
{
                 switch (menu())
	{
		case 1:
		//blahblah                 
                  break;
		case 2:
		//blahblah
		break;
		default:
		//blahblah                
         }
	
}

int menu()
{
	int x;
	cin>>x;

	while (true)
	{
		if (!cin)
		{
			cin.clear();
			cin.ignore( numeric_limits <streamsize> ::max(), '\n');
			cout << "\nThat was a letter. Letters are not numbers.\n\nTry Again:\n\n";
		
		}

		else
		{
			cin.ignore( numeric_limits <streamsize> ::max(), '\n');
			break;
		}
	}
	return x;
}

It's a bit cumbersome, but it got the job done.

EDIT: I had a hell of a time trying to get the source code box to format correctly (still looks crappy). It didnt copy well from my compiler. Sorry it's so...spacey...

[kempofighter]

FYI, this is another great link for stdio FAQs.
http://www.parashift.com/c++-faq-lite/input-output.html