# isnan

From cppreference.com
< c‎ | numeric‎ | math

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

nan nanf nanl (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

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