isnan

 

C

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Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

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Technical Specifications

 

Numerics

Common mathematical functions

Floating-point environment

Complex number arithmetics

Random number generation

Type-generic math

 

Common mathematical functions

Functions
Basic operations
abslabsllabsimaxabs (C99) fabs divldivlldivimaxdiv (C99) fmod	remainder (C99) remquo (C99) fma (C99) fmax (C99) fmin (C99) fdim (C99) nannanfnanl (C99)(C99)(C99)
Exponential functions
exp exp2 (C99) expm1 (C99)	log log10 log1p (C99) log2 (C99)
Power functions
sqrt cbrt (C99)	hypot (C99) pow
Trigonometric and hyperbolic functions
sin cos tan asin acos atan atan2	sinh cosh tanh asinh (C99) acosh (C99) atanh (C99)
Error and gamma functions
erf (C99) erfc (C99)	lgamma (C99) tgamma (C99)
Nearest integer floating point operations
ceil floor roundlroundllround (C99)(C99)(C99)	trunc (C99) nearbyint (C99) rintlrintllrint (C99)(C99)(C99)
Floating point manipulation functions
ldexp scalbnscalbln (C99)(C99) ilogb (C99) logb (C99)	frexp modf nextafternexttoward (C99)(C99) copysign (C99)
Classification
fpclassify (C99) isfinite (C99) isinf (C99) isnan (C99) isnormal (C99) signbit (C99)	isgreater (C99) isgreaterequal (C99) isless (C99) islessequal (C99) islessgreater (C99) isunordered (C99)
Types
div_tldiv_tlldiv_timaxdiv_t (C99)(C99)	float_tdouble_t (C99)(C99)
Macro constants
HUGE_VALFHUGE_VALHUGE_VALL (C99)(C99) FP_FAST_FMAFFP_FAST_FMAFP_FAST_FMAL (C99)(C99)(C99) math_errhandlingMATH_ERRNOMATH_ERRNOEXCEPT (C99)(C99)(C99)	INFINITY (C99) NAN (C99) FP_ILOGB0FP_ILOGBNAN (C99)(C99) FP_NORMALFP_SUBNORMALFP_ZEROFP_INFINITEFP_NAN (C99)(C99)(C99)(C99)(C99)

 

Defined in header <math.h>
#define isnan(arg) /* implementation defined */		(since C99)

Determines if the given floating point number arg is a not-a-number (NaN) value. The macro returns an integral value.

FLT_EVAL_METHOD is ignored: even if the argument is evaluated with more range and precision than its type, it is first converted to its semantic type, and the classification is based on that (this matters if the evaluation type supports NaNs, while the semantic type does not).

Parameters

arg

-

floating point value

Return value

Nonzero integral value if arg is a NaN, ​0​ otherwise.

Notes

There are many different NaN values with different sign bits and payloads, see nan.

NaN values never compare equal to themselves or to other NaN values. Copying a NaN may change its bit pattern.

Another way to test if a floating-point value is NaN is to compare it with itself: bool is_nan(double x) { return x != x; }

Example

Run this code

#include <stdio.h>
#include <math.h>
#include <float.h>
 
int main(void)
{
    printf("isnan(NAN)         = %d\n", isnan(NAN));
    printf("isnan(INFINITY)    = %d\n", isnan(INFINITY));
    printf("isnan(0.0)         = %d\n", isnan(0.0));
    printf("isnan(DBL_MIN/2.0) = %d\n", isnan(DBL_MIN/2.0));
    printf("isnan(0.0 / 0.0)   = %d\n", isnan(0.0/0.0));
    printf("isnan(Inf - Inf)   = %d\n", isnan(INFINITY - INFINITY));
}

Possible output:

isnan(NAN)         = 1
isnan(INFINITY)    = 0
isnan(0.0)         = 0
isnan(DBL_MIN/2.0) = 0
isnan(0.0 / 0.0)   = 1
isnan(Inf - Inf)   = 1

References

• C11 standard (ISO/IEC 9899:2011):

• 7.12.3.4 The isnan macro (p: 236-237)

• C99 standard (ISO/IEC 9899:1999):

• 7.12.3.4 The isnan macro (p: 217)

See also

nannanfnanl (C99)(C99)(C99)	returns a NaN (not-a-number) (function)
fpclassify (C99)	classifies the given floating-point value (function)
isfinite (C99)	checks if the given number has finite value (function)
isinf (C99)	checks if the given number is infinite (function)
isnormal (C99)	checks if the given number is normal (function)
isunordered (C99)	checks if two floating-point values are unordered (function)
C++ documentation for isnan