# isnan() Function with Example in C++

C++ isnan() function: Here, we are going to learn about the isnan() function with example of cmath header in C++ programming language?
Submitted by IncludeHelp, on May 17, 2020

## C++ isnan() function

isnan() function is a library function of cmath header, it is used to check whether the given value is a NaN (Not-A-Number). It accepts a value (float, double or long double) and returns 1 if the given value is NaN; 0, otherwise.

Syntax of isnan() function:

In C99, it has been implemented as a macro,

```    macro isnan(x)
```

In C++11, it has been implemented as a function,

```    bool isnan (float x);
bool isnan (double x);
bool isnan (long double x);
```

Parameter(s):

• x – represents a value to be checked as a NaN.

Return value:

The returns type of this function is bool, it returns 1 if the x is NaN; 0, otherwise.

Example:

```    Input:
float x = 0.0f/0.0f;

Function call:
isnan(x);

Output:
1

Input:
float x = sqrt(-1.0f);

Function call:
isnan(x);

Output:
1

Input:
float x = 10.0f;

Function call:
isnan(x);

Output:
0
```

### C++ code to demonstrate the example of isnan() function

```// C++ code to demonstrate the example of
// isnan() function

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

int main()
{
cout << "isnan(sqrt(-10.0f)): " << isnan(sqrt(-10.0f)) << endl;
cout << "isnan(0.0f/0.0f): " << isnan(0.0f / 0.0f) << endl;
cout << "isnan(0.0f/1.0f): " << isnan(0.0f / 1.0f) << endl;
cout << "isnan(1.0f/0.0f): " << isnan(1.0f / 0.0f) << endl;

float x = sqrt(-1.0f);

// checking using the condition
if (isnan(x)) {
cout << x << " is a NaN." << endl;
}
else {
cout << x << " is not a NaN." << endl;
}

x = sqrt(2);

if (isnan(x)) {
cout << x << " is a NaN." << endl;
}
else {
cout << x << " is not a NaN." << endl;
}

return 0;
}
```

Output

```isnan(sqrt(-10.0f)): 1
isnan(0.0f/0.0f): 1
isnan(0.0f/1.0f): 0
isnan(1.0f/0.0f): 0
-nan is a NaN.
1.41421 is not a NaN.
```

Reference: C++ isnan() function

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