NaN in C++ – What is it and How to Check For it?

NaN, an acronym for Not a Number is an exception that usually occurs in the cases when an expression results in a number that is undefined or can’t be represented. It is used for floating-point operations. For example:

• The square root of negative numbers
• Division by zero
• Taking the log of zero or a negative number etc.

CPP

// C++ code to demonstrate NaN exception
#include <cmath>
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    float a = 2, b = -2;

    // Prints the number (1.41421)
    cout << sqrt(a) << endl;

    // Prints "nan" exception
    // sqrt(-2) is complex number
    cout << sqrt(b) << endl;

    return 0;
}

Output

1.41421
-nan

How to check for NaN in C++?

Method 1: Using compare (“==”) operator.

In this method, we check if a number is complex by comparing it with itself. If the result is true, then the number is not complex i.e., real. But if the result is false, then “nan” is returned, i.e. the number is complex.

CPP

// C++ code to check for NaN exception
// using "==" operator
#include <cmath>
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    float a = sqrt(2);
    float b = sqrt(-2);

    // Returns true, a is real number
    // prints "Its a real number"
    a == a ? cout << "Its a real number" << endl
           : cout << "Its NaN" << endl;

    // Returns false, b is complex number
    // prints "Its nan"
    b == b ? cout << "Its a real number" << endl
           : cout << "Its NaN" << endl;

    return 0;
}

Output

Its a real number
Its NaN

Method 2: Using inbuilt function “isnan()”

Another way to check for NaN is by using “isnan()” function, this function returns true if a number is complex else it returns false. This C library function is present in <cmath> header file.

CPP

// C++ code to check for NaN exception
// using "isnan()"
#include <cmath>
#include <iostream>
using namespace std;

// Driver Code
int main()
{
    float a = sqrt(2);
    float b = sqrt(-2);

    // Returns false as a
    // is real number
    isnan(a) ? cout << "Its NaN" << endl
             : cout << "Its a real number" << endl;

    // Returns true as b is NaN
    isnan(b) ? cout << "Its NaN" << endl
             : cout << "Its a real number" << endl;

    return 0;
}

Output

Its a real number
Its NaN

This article is contributed by Manjeet Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.