C C NaN constant literal

Question

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

User · Answer

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)

User · Answer

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

User · Answer

As others have pointed out you are looking for std  numeric limits lt double gt   quiet NaN   although I have to say I prefer the cppreference com 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 std  numeric limits  has quiet NaN 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 here if true means your platform supports IEEE 754 standard  This previous thread explains this should be true for most situations

User · Answer

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  amp b  for giving the value of b to a if a  NULL     printf  quot NULL quot    else   printf  a    it is very simple and straitforward  it worked for me in Arduino IDE

User · Answer

In C  NAN is declared in  lt math h gt    In C    std  numeric limits lt double gt   quiet NaN   is declared in  lt limits gt    But for checking whether a value is NaN  you can t compare it with another NaN value  Instead use isnan   from  lt math h gt  in C  or std  isnan   from  lt cmath gt  in C

[c++] C/C++ NaN constant (literal)?

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