cpp nan value

1What is NaN ?
2NaN, acronym for “Not a Number” is an exception
3which usually occurs in the caseswhen an expression
4results in a number that can’t be represented.
5 For example square root of negative numbers.
6  
7  // C++ code to demonstrate NaN exception 
8#include<iostream> 
9#include<cmath> // for sqrt() 
10using namespace std; 
11int main() 
12{ 
13    float a = 2, b = -2; 
14  
15    // Prints the number (1.41421) 
16    cout << sqrt(a) << endl; 
17  
18    // Prints "nan" exception 
19    // sqrt(-2) is complex number 
20    cout << sqrt(b) << endl; 
21  
22    return 0; 
23} 
24Output:
25
261.41421
27-nan