Checking if a double or float is NaN in C

Question

Is there an isnan   function   PS   I m in MinGW  if that makes a difference    I had this solved by using isnan   from  lt math h gt   which doesn t exist in  lt cmath gt   which I was  includeing at first

User · Answer

You can use the isnan   function  but you need to include the C math library    include  lt cmath gt    As this function is part of C99  it is not available everywhere  If your vendor does not supply the function  you can also define your own variant for compatibility   inline bool isnan double x        return x    x

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According to the IEEE standard  NaN values have the odd property that comparisons involving them are always false  That is  for a float f  f    f will be true only if f is NaN    Note that  as some comments below have pointed out  not all compilers respect this when optimizing code    For any compiler which claims to use IEEE floating point  this trick should work  But I can t guarantee that it will work in practice  Check with your compiler  if in doubt

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It seems to me that the best truly cross-platform approach would be to use a union and to test the bit pattern of the double to check for NaNs    I have not thoroughly tested this solution  and there may be a more efficient way of working with the bit patterns  but I think that it should work    include  lt stdint h gt   include  lt stdio h gt   union NaN       uint64 t bits      double num      int main           Test if a double is NaN     double d   0 0   0 0      union NaN n      n num   d      if  n bits   0x800FFFFFFFFFFFFF     0xFFFFFFFFFFFFFFFF                printf  NaN   f   d              return 0

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This detects infinity and also NaN in Visual Studio by checking it is within double limits      include  lt float h gt  double x  y   -1 1  x   sqrt y   if  x  gt   DBL MIN  amp  amp  x  lt   DBL MAX       cout  lt  lt   DETECTOR-2 of errors FAILS   lt  lt  endl  else     cout  lt  lt   DETECTOR-2 of errors OK   lt  lt  endl

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There are three  official  ways  posix isnan macro  c  0x isnan function template  or visual c    isnan function   Unfortunately it s rather impractical to detect which of those to use   And unfortunately  there s no reliable way to detect whether you have IEEE 754 representation with NaNs  The standard library offers an official such way  numeric limits lt double gt   is iec559   But in practice compilers such as g   screw that up   In theory one could use simply x    x  but compilers such as g   and visual c   screw that up   So in the end  test for the specific NaN bitpatterns  assuming  and hopefully enforcing  at some point   a particular representation such as IEEE 754     EDIT  as an example of  compilers such as g    hellip  screw that up   consider   include  lt limits gt   include  lt assert h gt   void foo  double a  double b         assert  a    b       int main         typedef std  numeric limits lt double gt  Info      double const nan1   Info  quiet NaN        double const nan2   Info  quiet NaN        foo  nan1  nan2        Compiling with g    TDM-2 mingw32  4 4 1    C  test gt  type  C  Program Files  commands gnuc bat   rem -finput-charset windows-1252  g   -O -pedantic -std c  98 -Wall -Wwrite-strings    -Wno-long-long  C  test gt  gnuc x cpp  C  test gt  a  amp  amp  echo works       echo  failed works     C  test gt  gnuc x cpp --fast-math  C  test gt  a  amp  amp  echo works       echo  failed Assertion failed  a    b  file x cpp  line 6  This application has requested the Runtime to terminate it in an unusual way  Please contact the application s support team for more information   failed  C  test gt

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The IEEE standard says when the exponent is all 1s and the mantissa is not zero  the number is a NaN  Double is 1 sign bit  11 exponent bits and 52 mantissa bits  Do a bit check

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inline bool IsNan float f        const uint32 u     uint32   amp f      return  u amp 0x7F800000     0x7F800000  amp  amp   u amp 0x7FFFFF         Both NaN and qNan     inline bool IsNan double d        const uint64 u     uint64   amp d      return  u amp 0x7FF0000000000000ULL     0x7FF0000000000000ULL  amp  amp   u amp 0xFFFFFFFFFFFFFULL       This works if sizeof int  is 4 and sizeof long long  is 8   During run time it is only comparison  castings do not take any time  It just changes comparison flags configuration to check equality

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nan prevention My answer to this question is don t use retroactive checks for nan   Use preventive checks for divisions of the form 0 0 0 0 instead   include  lt float h gt  float x 0 f                  I m gonna divide by x  if   x                       Wait  Let me check if x is 0   x   FLT MIN                oh  since x was 0  i ll just make it really small instead  float y   0 f   x            whew   nan  didn t appear   nan results from the operation 0 f 0 f  or 0 0 0 0   nan is a terrible nemesis to the stability of your code that must be detected and prevented very carefully1   The properties of nan that are different from normal numbers   nan is toxic   5 nan nan  nan is not equal to anything  not even itself  nan    nan  nan not greater than anything  nan   gt  0  nan is not less than anything  nan   lt  0   The last 2 properties listed are counter-logical and will result in odd behavior of code that relies on comparisons with a nan number  the 3rd last property is odd too but you re probably not ever going to see x    x   in your code  unless you are checking for nan  unreliably     In my own code  I noticed that nan values tend to produce difficult to find bugs   Note how this is not the case for inf or -inf    -inf  lt  0  returns TRUE    0  lt  inf   returns TRUE  and even  -inf  lt  inf  returns TRUE  So  in my experience  the behavior of the code is often still as desired   what to do under nan What you want to happen under 0 0 0 0 must be handled as a special case  but what you do must depend on the numbers you expect to come out of the code  In the example above  the result of  0 f FLT MIN  will be 0  basically   You may want 0 0 0 0 to generate HUGE instead   So  float x 0 f  y 0 f  z  if   x  amp  amp   y         0 f 0 f case   z   FLT MAX        biggest float possible else   z   y x            regular division   So in the above  if x were 0 f  inf would result  which has pretty good nondestructive behavior as mentioned above actually   Remember  integer division by 0 causes a runtime exception   So you must always check for integer division by 0   Just because 0 0 0 0 quietly evaluates to nan doesn t mean you can be lazy and not check for 0 0 0 0 before it happens  1 Checks for nan via x    x are sometimes unreliable  x    x being stripped out by some optimizing compilers that break IEEE compliance  specifically when the -ffast-math switch is enabled

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There is an std  isnan if you compiler supports c99 extensions  but I m not sure if mingw does   Here is a small function which should work if your compiler doesn t have the standard function   bool custom isnan double var        volatile double d   var      return d    d

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You can use numeric limits lt float gt   quiet NaN    defined in the limits standard library to test with   There s a separate constant defined for double    include  lt iostream gt   include  lt math h gt   include  lt limits gt   using namespace std   int main         cout  lt  lt   The quiet NaN for type float is              lt  lt  numeric limits lt float gt   quiet NaN             lt  lt  endl      float f nan   numeric limits lt float gt   quiet NaN        if  isnan f nan                cout  lt  lt   Float was Not a Number     lt  lt  f nan  lt  lt  endl           return 0      I don t know if this works on all platforms  as I only tested with g   on Linux

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As of C  14 there are a number of ways to test if a floating point number value is a NaN   Of these ways  only checking of the bits of the number s representation  works reliably  as noted in my original answer  In particular  std  isnan and the often proposed check v    v  do not work reliably and should not be used  lest your code stops working correctly when someone decides that floating point optimization is needed  and asks the compiler to do that  This situation can change  compilers can get more conforming  but for this issue that hasn t happened in the 6 years since the original answer   For about 6 years my original answer was the selected solution for this question  which was OK  But recently a highly upvoted answer recommending the unreliable v    v test has been selected  Hence this additional more up-to-date answer  we now have the C  11 and C  14 standards  and C  17 on the horizon      The main ways to check for NaN-ness  as of C  14  are    std  isnan value    is the intended standard library way since C  11  isnan apparently conflicts with the Posix macro of the same name  but in practice that isn t a problem  The main problem is that when floating point arithmetic optimization is requested  then with at least one main compiler  namely g    std  isnan returns false for NaN argument   fpclassify value     FP NAN    Suffers from the same problem as std  isnan  i e   is not reliable   value    value    Recommended in many SO answers  Suffers from the same problem as std  isnan  i e   is not reliable   value    Fp info  quiet NaN      This is a test that with standard behavior should not detect NaNs  but that with the optimized behavior maybe could detect NaNs  due to optimized code just comparing the bitlevel representations directly   and perhaps combined with another way to cover the standard un-optimized behavior  could reliably detect NaN  Unfortunately it turned out to not work reliably   ilogb value     FP ILOGBNAN    Suffers from the same problem as std  isnan  i e   is not reliable  isunordered 1 2345  value    Suffers from the same problem as std  isnan  i e   is not reliable  is ieee754 nan  value     This isn t a standard function  It s checking of the bits according to the IEEE 754 standard  It s completely reliable but the code is somewhat system-dependent      In the following complete test code  ldquo success rdquo  is whether an expression reports Nan-ness of the value  For most expressions this measure of success  the goal of detecting NaNs and only NaNs  corresponds to their standard semantics  For the  value    Fp info  quiet NaN      expression  however  the standard behavior is that it doesn t work as a NaN-detector    include  lt cmath gt            std  isnan  std  fpclassify  include  lt iostream gt   include  lt iomanip gt          std  setw  include  lt limits gt   include  lt limits h gt         CHAR BIT  include  lt sstream gt   include  lt stdint h gt         uint64 t using namespace std    define TEST  x  expr  expected           amp                const auto value   x            const bool result   expr            ostringstream stream            stream  lt  lt  boolalpha  lt  lt   x        lt  lt  x  lt  lt         expr         lt  lt  result            cout                lt  lt  setw  60    lt  lt  stream str    lt  lt                      lt  lt   result    expected   Success     FAILED                  lt  lt  endl              define TEST ALL VARIABLES  expression         TEST  v  expression  true          TEST  u  expression  false          TEST  w  expression  false    using Fp info   numeric limits lt double gt    inline auto is ieee754 nan  double const x       - gt  bool       static constexpr bool   is claimed ieee754    Fp info  is iec559      static constexpr int    n bits per byte       CHAR BIT      using Byte   unsigned char       static assert  is claimed ieee754             static assert  n bits per byte    8             static assert  sizeof  x      sizeof  uint64 t                 ifdef  MSC VER         uint64 t const bits   reinterpret cast lt uint64 t const amp  gt   x         else         Byte bytes sizeof x            memcpy  bytes   amp x  sizeof  x              uint64 t int value          memcpy   amp int value  bytes  sizeof  x              uint64 t const amp  bits   int value       endif      static constexpr uint64 t   sign mask         0x8000000000000000      static constexpr uint64 t   exp mask          0x7FF0000000000000      static constexpr uint64 t   mantissa mask     0x000FFFFFFFFFFFFF        void  sign mask      return  bits  amp  exp mask     exp mask and  bits  amp  mantissa mask     0     auto main       - gt  int       double const v   Fp info  quiet NaN        double const u   3 14      double const w   Fp info  infinity         cout  lt  lt  boolalpha  lt  lt  left      cout  lt  lt   Compiler claims IEEE 754      lt  lt  Fp info  is iec559  lt  lt  endl      cout  lt  lt  endl       TEST ALL VARIABLES  std  isnan value                        cout  lt  lt  endl      TEST ALL VARIABLES   fpclassify value     FP NAN            cout  lt  lt  endl      TEST ALL VARIABLES   value    value                         cout  lt  lt  endl      TEST ALL VARIABLES   value    Fp info  quiet NaN            cout  lt  lt  endl      TEST ALL VARIABLES   ilogb value     FP ILOGBNAN            cout  lt  lt  endl      TEST ALL VARIABLES  isunordered 1 2345  value               cout  lt  lt  endl      TEST ALL VARIABLES  is ieee754 nan  value          Results with g    note again that the standard behavior of  value    Fp info  quiet NaN    is that it doesn t work as a NaN-detector  it s just very much of practical interest here      C  my forums so 282   detect NaN    gt  g   --version   find      g    x86 64-win32-sjlj-rev1  Built by MinGW-W64 project  6 3 0   C  my forums so 282   detect NaN    gt  g   foo cpp  amp  amp  a Compiler claims IEEE 754   true  v   nan   std  isnan value     true                           Success u   3 14   std  isnan value     false                         Success w   inf   std  isnan value     false                          Success  v   nan    fpclassify value     0x0100     true               Success u   3 14    fpclassify value     0x0100     false             Success w   inf    fpclassify value     0x0100     false              Success  v   nan    value    value     true                            Success u   3 14    value    value     false                          Success w   inf    value    value     false                           Success  v   nan    value    Fp info  quiet NaN       false            FAILED u   3 14    value    Fp info  quiet NaN       false           Success w   inf    value    Fp info  quiet NaN       false            Success  v   nan    ilogb value       int 0x80000000      true         Success u   3 14    ilogb value       int 0x80000000      false       Success w   inf    ilogb value       int 0x80000000      false        Success  v   nan   isunordered 1 2345  value     true                  Success u   3 14   isunordered 1 2345  value     false                Success w   inf   isunordered 1 2345  value     false                 Success  v   nan   is ieee754 nan  value      true                     Success u   3 14   is ieee754 nan  value      false                   Success w   inf   is ieee754 nan  value      false                    Success   C  my forums so 282   detect NaN    gt  g   foo cpp -ffast-math  amp  amp  a Compiler claims IEEE 754   true  v   nan   std  isnan value     false                          FAILED u   3 14   std  isnan value     false                         Success w   inf   std  isnan value     false                          Success  v   nan    fpclassify value     0x0100     false              FAILED u   3 14    fpclassify value     0x0100     false             Success w   inf    fpclassify value     0x0100     false              Success  v   nan    value    value     false                           FAILED u   3 14    value    value     false                          Success w   inf    value    value     false                           Success  v   nan    value    Fp info  quiet NaN       true             Success u   3 14    value    Fp info  quiet NaN       true            FAILED w   inf    value    Fp info  quiet NaN       true             FAILED  v   nan    ilogb value       int 0x80000000      true         Success u   3 14    ilogb value       int 0x80000000      false       Success w   inf    ilogb value       int 0x80000000      false        Success  v   nan   isunordered 1 2345  value     false                 FAILED u   3 14   isunordered 1 2345  value     false                Success w   inf   isunordered 1 2345  value     false                 Success  v   nan   is ieee754 nan  value      true                     Success u   3 14   is ieee754 nan  value      false                   Success w   inf   is ieee754 nan  value      false                    Success   C  my forums so 282   detect NaN    gt      Results with Visual C       C  my forums so 282   detect NaN    gt  cl  nologo- 2 gt  1   find      Microsoft  R  C C   Optimizing Compiler Version 19 00 23725 for x86   C  my forums so 282   detect NaN    gt  cl foo cpp  Feb  amp  amp  b foo cpp Compiler claims IEEE 754   true  v   nan   std  isnan value     true                           Success u   3 14   std  isnan value     false                         Success w   inf   std  isnan value     false                          Success  v   nan    fpclassify value     2     true                    Success u   3 14    fpclassify value     2     false                  Success w   inf    fpclassify value     2     false                   Success  v   nan    value    value     true                            Success u   3 14    value    value     false                          Success w   inf    value    value     false                           Success  v   nan    value    Fp info  quiet NaN       false            FAILED u   3 14    value    Fp info  quiet NaN       false           Success w   inf    value    Fp info  quiet NaN       false            Success  v   nan    ilogb value     0x7fffffff     true                Success u   3 14    ilogb value     0x7fffffff     false              Success w   inf    ilogb value     0x7fffffff     true                FAILED  v   nan   isunordered 1 2345  value     true                  Success u   3 14   isunordered 1 2345  value     false                Success w   inf   isunordered 1 2345  value     false                 Success  v   nan   is ieee754 nan  value      true                     Success u   3 14   is ieee754 nan  value      false                   Success w   inf   is ieee754 nan  value      false                    Success   C  my forums so 282   detect NaN    gt  cl foo cpp  Feb  fp fast  amp  amp  b foo cpp Compiler claims IEEE 754   true  v   nan   std  isnan value     true                           Success u   3 14   std  isnan value     false                         Success w   inf   std  isnan value     false                          Success  v   nan    fpclassify value     2     true                    Success u   3 14    fpclassify value     2     false                  Success w   inf    fpclassify value     2     false                   Success  v   nan    value    value     true                            Success u   3 14    value    value     false                          Success w   inf    value    value     false                           Success  v   nan    value    Fp info  quiet NaN       false            FAILED u   3 14    value    Fp info  quiet NaN       false           Success w   inf    value    Fp info  quiet NaN       false            Success  v   nan    ilogb value     0x7fffffff     true                Success u   3 14    ilogb value     0x7fffffff     false              Success w   inf    ilogb value     0x7fffffff     true                FAILED  v   nan   isunordered 1 2345  value     true                  Success u   3 14   isunordered 1 2345  value     false                Success w   inf   isunordered 1 2345  value     false                 Success  v   nan   is ieee754 nan  value      true                     Success u   3 14   is ieee754 nan  value      false                   Success w   inf   is ieee754 nan  value      false                    Success   C  my forums so 282   detect NaN    gt      Summing up the above results  only direct testing of the bit-level representation  using the is ieee754 nan function defined in this test program  worked reliably in all cases with both g   and Visual C       Addendum  After posting the above I became aware of yet another possible to test for NaN  mentioned in another answer here  namely   value  lt  0      value  gt   0    That turned out to work fine with Visual C   but failed with g   s -ffast-math option  Only direct bitpattern testing works reliably

User · Answer

As comments above state a    a will not work in g   and some other compilers  but this trick should  It may not be as efficient  but it s still a way   bool IsNan float a        char s 4       sprintf s     3f   a       if  s 0    n   return true      else return false      Basically  in g    I am not sure about others though  printf prints  nan  on  d or   f formats if variable is not a valid integer float  Therefore this code is checking for the first character of string to be  n   as in  nan

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First solution  if you are using C  11  Since this was asked there were a bit of new developments  it is important to know that std  isnan   is part of C  11  Synopsis  Defined in header  lt cmath gt   bool isnan  float arg     since C  11  bool isnan  double arg     since C  11  bool isnan  long double arg     since C  11    Determines if the given floating point number arg is not-a-number  NaN    Parameters  arg  floating point value  Return value  true if arg is NaN  false otherwise  Reference  http   en cppreference com w cpp numeric math isnan  Please note that this is incompatible with -fast-math if you use g    see below for other suggestions     Other solutions  if you using non C  11 compliant tools  For C99  in C  this is implemented as a macro isnan c that returns an int value  The type of x shall be float  double or long double   Various vendors may or may not include or not a function isnan     The supposedly portable way to check for NaN is to use the IEEE 754 property that NaN is not equal to itself  i e  x    x will be false for x being NaN   However the last option may not work with every compiler and some settings  particularly optimisation settings   so in last resort  you can always check the bit pattern

User · Answer

The following code uses the definition of NAN  all exponent bits set  at least one fractional bit set  and assumes that sizeof int    sizeof float    4   You can look up NAN in Wikipedia for the details   bool IsNan  float value         return     UINT   amp value   amp  0x7fffffff   gt  0x7f800000

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Considering that  x    x  is not always guaranteed for NaN  such as if using the -ffast-math option   I ve been using    define IS NAN x     x   lt  0       x   gt   0     Numbers can t be both  lt  0 and    0  so really this check only passes if the number is neither less than  nor greater than or equal to zero   Which is basically no number at all  or NaN   You could also use this if you prefer    define IS NAN x      x  lt 0   amp  amp     x  gt  0    I m not sure how this is affected by -ffast-math though  so your mileage may vary

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As for me the solution could be a macro to make it explicitly inline and thus fast enough  It also works for any float type  It bases on the fact that the only case when a value is not equals itself is when the value is not a number    ifndef isnan    define isnan a   a    a   endif

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This works    include  lt iostream gt   include  lt math h gt  using namespace std   int main        char ch  a     double val   nan  amp ch     if isnan val        cout  lt  lt   isnan   lt  lt  endl     return 0      output  isnan

User · Answer

There is no isnan   function available in current C   Standard Library  It was introduced in C99 and defined as a macro not a function  Elements of standard library defined by C99 are not part of current C   standard ISO IEC 14882 1998 neither its update ISO IEC 14882 2003   In 2005 Technical Report 1 was proposed  The TR1 brings compatibility with C99 to C    In spite of the fact it has never been officially adopted to become C   standard  many  GCC 4 0  or Visual C   9 0  C   implementations do provide TR1 features  all of them or only some  Visual C   9 0 does not provide C99 math functions    If TR1 is available  then cmath includes C99 elements like isnan    isfinite    etc  but they are defined as functions  not macros  usually in std  tr1   namespace  though many implementations  i e  GCC 4  on Linux or in XCode on Mac OS X 10 5   inject them directly to std    so std  isnan is well defined   Moreover  some implementations of C   still make C99 isnan   macro available for C    included through cmath or math h   what may cause more confusions and developers may assume it s a standard behaviour   A note about Viusal C    as mentioned above  it does not provide std  isnan neither std  tr1  isnan  but it provides an extension function defined as  isnan   which has been available since Visual C   6 0  On XCode  there is even more fun  As mentioned  GCC 4  defines std  isnan  For older versions of compiler and library form XCode  it seems  here is relevant discussion   haven t had chance to check myself  two functions are defined    inline isnand   on Intel and   isnand   on Power PC

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There is also a header-only library present in Boost that have neat tools to deal with floating point datatypes   include  lt boost math special functions fpclassify hpp gt    You get the following functions   template  lt class T gt  bool isfinite T z   template  lt class T gt  bool isinf T t   template  lt class T gt  bool isnan T t   template  lt class T gt  bool isnormal T t     If you have time then have a look at whole Math toolkit from Boost  it has many useful tools and is growing quickly   Also when dealing with floating and non-floating points it might be a good idea to look at the Numeric Conversions

User · Answer

A possible solution that would not depend on the specific IEEE representation for NaN used would be the following   template lt class T gt  bool isnan  T f         T  nan     T 0 0  T 0 0      return 0    memcmp   void   amp f   void   amp  nan  sizeof T

[c++] Checking if a double (or float) is NaN in C++

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