C++ input validation for numeric input

Question

I'm creating my own very simple program that allows the user to input a numeric value. At the moment the code works just fine, but I need a validation if else statement. This is what I have at the moment;

#include <iostream>
#include <string>

using namespace std;

int main()
{

    unsigned __int64 input = 0;
    char str[] = "qwertyuiopasdfghjklzxcvbnm[]{};'#:@~,./<>?|!£$%^&*()";

    cout << "Insert a number" << endl;
    cin >> input;

    if (input % 2 == 0) 
    {
        cout << "Even!" << endl;
    }
    else 
    {
        if (input% 2 == 1)
        {
            cout << "Odd" << endl;
            cout << "Lets make it even shall we? " << "Your new number is... " << input + 1 << endl;
        }
        else if (isdigit(str[0]))
        {
            cout << "That isn't a number!" << endl;
        }
    }

    system("pause");
    return 0;

}

The issue I am having is that if the user inputs anything that isn't a number, the value it returns is "Even".

I hope you guys and girls can help! John

If you enter anything but a number, nothing is extracted from the stream and input stays 0. That assumes "anything but a number" does not include 123abc. — Qaz
– Qaz, Commented Nov 18, 2012 at 22:14
You need to check for failure when you read input. parashift.com/c++-faq/istream-and-ignore.html — Qaz
– Qaz, Commented Nov 18, 2012 at 22:18

Kerrek SB · Accepted Answer · 2012-11-18 22:36:06Z

4

Don't use token extraction (>>) for primary parsing. Once the extraction fails, your primary input is left in an unspecified state, which is terrible. Instead, read the input line by line and then process each line.

Also, never ignore the result of an input operation. That's just a flat error.

So, putting all this together, here's how you could handle this:

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

int main()
{
    for (std::string line; std::cout << "Input: " && std::getline(std::cin, line); )
    {
        std::cout << "We're parsing your line '" << line << "'\n";

        int n;
        std::istringstream iss(line);

        if (iss >> n >> std::ws && iss.get() == EOF)
        {
            std::cout << "It was a number: " << n << "\n";
        }
        else if (line.empty())
        {
            std::cout << "You didn't say anything!\n";
        }
        else
        {
            std::cout << "We could not parse '" << line << "' as a number.\n";
        }
    }

    std::cout << "Goodbye!\n";
}

Note that all input operations (namely >> and getline) appear in immediate boolean contexts!

edited Nov 18, 2012 at 22:36

answered Nov 18, 2012 at 22:19

Kerrek SB

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7 Comments

John Brown Over a year ago

Thanks,but I don't really understand this code. I've been doing c++ for seven weeks now!

Kerrek SB Over a year ago

@JohnBrown: Then your homework for week 8 is to understand this code!

Kerrek SB Over a year ago

@JohnBrown: Glad you like it! More importantly, you're now in a position to answer 20% of all C++ questions on SO!

John Brown Over a year ago

What variable name is being used for the input?

Kerrek SB Over a year ago

@JohnBrown: It's line, isn't it?

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