Banning non-numbers for user input and letting the user retry an input

[WowICantProgram]

How can I "ban" letters from being used while only letting numbers to get through? Inputting letters causes my program to crash. And if they were to input a letter or foreign character, how can I let them retry?

[LB]

To avoid screwing up your one and only source of input (std::cin), you should read whole lines at a time and then deal with them after that:

int x;
std::string line;
while((std::cout << "Enter a whole number: ") //prompt for input
   && std::getline(std::cin, line) //read whole line (almost never fails)
   && !(std::istringstream{line} >> x)) //validate input
{
    std::cout << "Invalid input, please try again." << std::endl;
}

Don't forget to #include <sstream>

[WowICantProgram]

Thanks! I've been trying to figure that out for a while now. I do have another question. How would I be able to manipulate the number that the user enters in? The tip you gave me worked, I just don't know how to transfer it to this because it ends after I enter a number

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
    double N1 = 0;
    double N2 = 0;
    double quotient = 0;

    cout<< "Enter first number:   "; // user input
    cin >> N1;
    cout<< "Enter second number:  ";
    cin >> N2;

    quotient = N1 / N2; // formula


   if (N2 == 0) // if divided by zero
    {
        cout << "Quotient does not exist" << endl;
    }
    else
    {
       cout << "Quotient is " << quotient << endl;
    }

return 0;
}

[LB]

You could take the code from above and put it into a function, then just call the function whenever you want input.

[WowICantProgram]

Sorry, I found that confusing. Could you further explain what you mean? I'm fairly new at this. I think only allowing digits 0-9, ".", and "-". would work but I think I'm getting too far ahead of myself for someone who just started.

[LB]

Don't worry about which characters make up a number - C++ handles that for you.

double ask_for_number(std::string const &prompt)
{
    double x;
    std::string line;
    while((std::cout << prompt << ": ") //prompt for input
       && std::getline(std::cin, line) //read whole line (almost never fails)
       && !(std::istringstream{line} >> x)) //validate input
    {
        std::cout << "Invalid input, please try again." << std::endl;
    }
}

int n1 = ask_for_number("Enter a number: ");
int n2 = ask_for_number("Enter another number: ");

[WowICantProgram]

Thanks again, LB! I couldn't get it to work properly with the code that you provided me so I made some minor changes to it. Is my code proper still, or did I just get lucky?

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
    double quotient = 0;

    double ask_for_number_1;
    string line;

    while((cout  << "Enter your first number: ") //prompt for input
       && getline(cin, line) //read whole line (almost never fails)
       && !(istringstream{line} >> ask_for_number_1)) //validate input
    {
        cout << "Invalid input, please try again." << endl;
    }
    double ask_for_number_2;
    string line2;
    while((cout  << "Enter your second number: ") 
       && getline(cin, line)
       && !(istringstream{line} >> ask_for_number_2))
    {
        cout << "Invalid input, please try again." << endl;
    }


    double N1 = ask_for_number_1;
    double N2 = ask_for_number_2;




    quotient = N1 / N2; // formula


   if (N2 == 0) // if divided by zero
    {
        cout << "Quotient does not exist" << endl;
    }
    else
    {
       cout << "Quotient is " << quotient << endl;
    }

return 0;
}

[LB]

Oops, the reason my code didn't work was because I forgot the return x; at the end:
http://ideone.com/8SizAy

You don't need your line2 variable on line 19 - the point of the line variable is just to hold the most recent line of input, so re-using it is intended.

Also, why not just call the variables N1 and N2 from the start? Then you wouldn't need lines 28 and 29.