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

• Difficulty Level : Easy
• Last Updated : 06 Jan, 2022

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 ` `#include ` `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 ` `#include ` `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 ` `#include ` `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```