C/C++ NaN constant (literal)?

Is this possible to assign a NaN to a double or float in C/C++? Like in JavaScript you do: a = NaN. So later you can check if the variable is a number or no.

In C, NAN is declared in <math.h>.

In C++, std::numeric_limits<double>::quiet_NaN() is declared in <limits>.

But for checking whether a value is NaN, you can't compare it with another NaN value. Instead use isnan() from <math.h> in C, or std::isnan() from <cmath> in C++.

As others have pointed out you are looking for std::numeric_limits<double>::quiet_NaN() although I have to say I prefer the http://en.cppreference.com/w/cpp/types/numeric_limits/quiet_NaN">cppreference.com</a> documents. Especially because this statement is a little vague:

>Only meaningful if std::numeric_limits::has_quiet_NaN == true.

and it was simple to figure out what this means on this site, if you check their section on http://en.cppreference.com/w/cpp/types/numeric_limits/has_quiet_NaN">`std::numeric_limits::has_quiet_NaN`</a> it says:

>This constant is meaningful for all floating-point types and is guaranteed to be true if std::numeric_limits::is_iec559 == true.

which as explained http://en.cppreference.com/w/cpp/types/numeric_limits/is_iec559">here</a> if true means your platform supports IEEE 754 standard. This https://stackoverflow.com/questions/2234468/do-any-real-world-cpus-not-use-ieee-754">previous thread explains this should be true for most situations.

This can be done using the numeric_limits in C++:

http://www.cplusplus.com/reference/limits/numeric_limits/

These are the methods you probably want to look at:

infinity()	T	Representation of positive infinity, if available.
quiet_NaN()	T	Representation of quiet (non-signaling) "Not-a-Number", if available.
signaling_NaN()	T	Representation of signaling "Not-a-Number", if available.

> Is this possible to assign a NaN to a double or float in C ...?

Yes, since C99, (C++11) <math.h> offers the below functions:

#include <math.h>
double nan(const char *tagp);
float nanf(const char *tagp);
long double nanl(const char *tagp);

which are like their strtod("NAN(n-char-sequence)",0) counterparts and NAN for assignments.

// Sample C code
uint64_t u64;
double x;
x = nan("0x12345");
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);
x = -strtod("NAN(6789A)",0);
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);
x = NAN;
memcpy(&u64, &x, sizeof u64); printf("(%" PRIx64 ")\n", u64);

Sample output: (Implementation dependent)

(7ff8000000012345)
(fff000000006789a)
(7ff8000000000000)

---

> ... check if the variable is a number or no.

Use isnan(), std::isnan() from <math.h>, <cmath>.

yes, by the concept of pointer you can do it like this for an int variable:

int *a;
int b=0;
a=NULL; // or a=&b; for giving the value of b to a
if(a==NULL) 
  printf("NULL");
else
  printf(*a);

it is very simple and straitforward. it worked for me in Arduino IDE.