Factorial program problem

Hi I am a beginner and I wrote this program and everything works except when I input a letter instead of a number the program crashes any suggestions on how I can fix that and also any feedback on my program would be really appreciated.

Code:

//Programmer: Fernando B
//Program: Factorial
//Purpose: factor a number      //need a boolean statement if the input is a letter??
//Formula 0! = 1
//        n! = n * (n-1)!
// Date: 12/21/10

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

long long factorial (long long n)
{
    if (n == 0) return 1;
    return n * factorial (n-1);
}

int main ()

{

    bool quit = false;
    while ( !quit)

    {
    long long n;
    cout << "Enter a positive integer to compute the factorial of: ";
    cin >> n;



    if (n <= 0)

    {
        cout << "Try again, please enter a positive integer!!!\a" << endl;

    }

    else if ( n >= 1)

    {
        char inputCharacter = 0;
        cout << n << "! =" << factorial(n) << endl;
        cout << "Would you like to try a new number? (Y)es (N)o" << endl;
        cin >> inputCharacter;

        if ( inputCharacter == 'Y' || inputCharacter == 'y')
        {
            quit = false;
        }

        else if ( inputCharacter == 'N' || inputCharacter == 'n')
        {
                cout << "Quitting...";
                quit = true;
        }


    }



    }


    return 0;
    system ("pause");


}

cin >> is formatted input. If the input doesn't match the format, then >> is the wrong thing to do. Look at the FAQ which exactly matches your question "Why does my program enter an infinite loop if the user inputs invalid data? (C++) "

cin >> is formatted input. If the input doesn't match the format, then >> is the wrong thing to do. Look at the FAQ which exactly matches your question "Why does my program enter an infinite loop if the user inputs invalid data? (C++) "

Thanks a lot tabstop I will be spending some time at the FAQ from now on, so here is my finished program...

Code:

//Programmer: Fernando B
//Program: Factorial
//Purpose: factor a number      //need a boolean statement if the input is a letter??
//Formula 0! = 1
//        n! = n * (n-1)!
// Date: 12/21/10

#include <iostream>
#include <ios>
#include <cmath>
#include <limits>

using namespace std;

long long factorial (long long n)
{
    if (n == 0) return 1;
    return n * factorial (n-1);
}

int main ()

{

    bool quit = false;
        while (!quit)

    {

    long long n;
    cout << "Enter a positive integer to compute the factorial of: ";
    while (!( cin >> n )){
    cin.clear(); // clear error state
    cin.ignore(numeric_limits <streamsize>::max(), '\n'); // remove the unrecognized char
    cout << "Invalid data input. Please try again: \a";
    }
    
    if (n <= 0)

    {
       cout << "Try again, enter a positive integer! \a" << endl;
    }

    else if ( n >= 1)

    {
        char inputCharacter = 0;
        cout << n << "! =" << factorial(n) << endl;
        cout << "Would you like to try a new number? (Y)es (N)o" << endl;
        cin >> inputCharacter;

        if ( inputCharacter == 'Y' || inputCharacter == 'y')
        {
            quit = false;
        }

        else if ( inputCharacter == 'N' || inputCharacter == 'n')
        {
                cout << "Quitting...";
                quit = true;
        }
        
    }
    }


    return 0;
    system ("pause");


}

So as it turns out I still had one more bug in the program when I asked to enter Y or N, the user could input any character and still go on... so here is the fix for that

Code:

/*Programmer: Fernando B
Program: Factorial
Purpose: factor a number
Formula 0! = 1
        n! = n * (n-1)!
 Date: 12/21/10
 notes with long long I can only go as far as 65*/

#include <iostream>
#include <ios>
#include <cmath>
#include <limits>

using namespace std;

long long factorial (long long  n)
{
    if (n == 0) return 1;
    return n * factorial (n-1);
}

int main ()

{

    bool quit = false;
        while (!quit)

    {

    long long  n;
    cout << "Enter a positive integer to compute the factorial of: ";
    while (!( cin >> n )){
    cin.clear(); // clear error state
    cin.ignore(numeric_limits <streamsize>::max(), '\n'); // remove the unrecognized char
    cout << "Invalid data input. Please try again: \a";
    }

    if (n <= 0)

    {
       cout << "Try again, enter a positive integer! \a" << endl;
    }

    else if ( n >= 1)

    {
        cout << n << "! =" << factorial(n) << endl;

    }

    {
        bool quit1 = false;
        while(!quit1)



        {


        char inputCharacter = 0;
        cout << "Would you like to try a new number? (Y)es (N)o" << endl;
        cin >> inputCharacter;




        if (!( inputCharacter == 'Y' || inputCharacter == 'y' || inputCharacter == 'N' || inputCharacter == 'n'))
        {
            cout << "Enter Y or N, try again! \a" << endl;
            quit1 = false;
        }


        else if ( inputCharacter == 'Y' || inputCharacter == 'y')
        {
            quit1 = true;

        }

        else if ( inputCharacter == 'N' || inputCharacter == 'n')
        {
            cout << "Quitting...";
            quit1 = true;
            quit = true;

        }





        }
    }




}

    return 0;
    system ("pause");

}