Input Validation of int C++

Question

I basically want to validate that I have an int and not a floating point number. What I currently have is:

int den1;
cout << "Enter denominator of first fraction" << endl;
cin >> den1;
while (den1 == 0){
   cout << "Enter a non-zero denominator" << endl;
   cin >> den1;
}

Is there a "test" to generate a boolean value for den1 == int? I'm trying to avoid using getline() because I don't want to use a string if it isn't necessary.

Answer 1

If you want to force your input to be of an integer type, then use an integer type for your input. If den1 is an int , it will not let you put a floating point value in it. That is, cin >> den1 will be an int value. If the user tries to input 3.14159 , only the 3 will be read (it will stop reading at the . . Note that the rest of the buffer will contain numbers as well, so if you don't clear it, the next attempt to read an integer will read 14159 .

EDIT

If you want to "force" the user to enter a valid integer, you can do something like this:

std::string line;

int value = 0;
bool valid = false;
do
{
    if (std::getline(std::cin, line))
    {
        if (std::string::npos == line.find('.'))
        {
            // no decimal point, so not floating point number
            value = std::stol(line);
            valid = true;
        }
        else
        {
            std::cin.clear();
        }
    }
} while (!valid);

Which is a lot of extra code compared to:

int value;
std::cin >> value;

Answer 2

You want to use something like

if (std::cin >> den) {
    // process den
}
else {
    // deal with invalid input
}

When an input operation fails, it sets std::ios_base::failbit on the stream and the stream converts to false instead of true . While the stream is in this failure mode, it won't read anything from the stream, ie, the failure mode as to be cleared, eg, using

std::cin.clear();

Once the failure mode is cleared, the offending character still sits in the stream. You can ignore the next character using, eg

std::cin.ignore();

or ignore all characters until the next newline:

std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');

Answer 3

First of all, user input is always a string. Next, you need to define your goal more precisely. For example a reasonable thing to distinguish is whether the input can be parsed in its entirety as an integer, or as a floating point number, or neither. Here's one way to do this with iostreams, disregarding whitespace:

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

for (std::string line; std::getline(std::cin, line); )
{
    std::istringstream iss1(line), iss2(line);
    int n;
    double x;

    if (iss1 >> n >> std::ws && iss1.get() == EOF)
    {
        // have an int, use "n"
    }
    else if (iss2 >> d >> std::ws && iss2.get() == EOF)
    {
        // have a floating point number, use "d"
    }
    else
    {
        // failed to parse the input
        continue;
    }
}