是否存在isnan()函数?
注:我在MinGW(如果这有区别的话)。
我使用isnan()从<math.h>解决了这个问题，这在<cmath>中不存在，我一开始是#包括在内的。
当前回答
南预防
我对这个问题的回答是不要对nan使用追溯检查。取而代之的是对表单0.0/0.0的划分使用预防性检查。
#include <float.h>
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是0.f/0运算的结果。F，或0.0/0.0。Nan是代码稳定性的一个可怕的克星，必须非常小心地检测和防止1。nan不同于普通数的特性:
Nan是有毒的，(5* Nan = Nan) Nan不等于任何东西，甚至不等于它本身(Nan != Nan) Nan不大于任何东西(Nan !> 0) Nan不小于任何值(Nan !< 0)
最后列出的2个属性是反逻辑的，将导致依赖于与nan数比较的代码的奇怪行为(最后3个属性也很奇怪，但您可能永远不会看到x != x ?)在你的代码中(除非你在检查nan(不可靠))。
在我自己的代码中，我注意到nan值往往会产生难以发现的错误。(请注意，这不是inf或-inf的情况。(-inf < 0)返回TRUE， (0 < inf)返回TRUE，甚至(-inf < inf)返回TRUE。因此，在我的经验中，代码的行为通常仍然是理想的)。
在奶奶手下该怎么办
您希望在0.0/0.0下发生的事情必须作为特殊情况处理，但是您所做的事情必须取决于您期望从代码中得到的数字。
在上面的例子中，(0.f/FLT_MIN)的结果基本上是0。你可能想让0.0/0.0生成HUGE。所以,
float x=0.f, y=0.f, z;
if( !x && !y ) // 0.f/0.f case
z = FLT_MAX ; // biggest float possible
else
z = y/x ; // regular division.
在上面，如果x = 0。F, inf会导致(实际上如上所述，它具有相当好的/非破坏性行为)。
记住，整数除以0会导致运行时异常。所以你必须总是检查整数除以0。仅仅因为0.0/0.0悄悄地计算为nan并不意味着您可以偷懒，在它发生之前不检查0.0/0.0。
通过x != x检查nan有时是不可靠的(x != x被一些破坏IEEE遵从性的优化编译器剥离，特别是当- fast-math开关启用时)。
其他回答
Boost中还提供了一个仅头文件的库，该库具有处理浮点数据类型的简洁工具
#include <boost/math/special_functions/fpclassify.hpp>
你会得到以下函数:
template <class T> bool isfinite(T z);
template <class T> bool isinf(T t);
template <class T> bool isnan(T t);
template <class T> bool isnormal(T t);
如果你有时间，那么看看Boost的整个数学工具包，它有许多有用的工具，并且正在快速增长。
此外，当处理浮点和非浮点时，查看数字转换可能是一个好主意。
如此:
#include <iostream>
#include <math.h>
using namespace std;
int main ()
{
char ch='a';
double val = nan(&ch);
if(isnan(val))
cout << "isnan" << endl;
return 0;
}
输出:isnan
截至c++ 14，有许多方法来测试浮点数值是否为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.
在大约6年的时间里，我最初的答案是这个问题的选择解决方案，这是OK的。但最近，一个高度好评的答案推荐了不可靠的v != v测试。因此，这是一个额外的最新答案(我们现在有c++ 11和c++ 14标准，以及即将出现的c++ 17标准)。
从c++ 14开始，检查nan属性的主要方法是:
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.
在下面完整的测试代码中，“success”是表达式是否报告值的Nan-ness。对于大多数表达式来说，这种成功的度量，即检测nan(且仅检测nan)的目标与它们的标准语义相对应。然而，对于(value == Fp_info::quiet_NaN())表达式，标准行为是它不能作为nan检测器工作。
#include <cmath> // std::isnan, std::fpclassify
#include <iostream>
#include <iomanip> // std::setw
#include <limits>
#include <limits.h> // CHAR_BIT
#include <sstream>
#include <stdint.h> // uint64_t
using namespace std;
#define TEST( x, expr, expected ) \
[&](){ \
const auto value = x; \
const bool result = expr; \
ostringstream stream; \
stream << boolalpha << #x " = " << x << ", (" #expr ") = " << result; \
cout \
<< setw( 60 ) << stream.str() << " " \
<< (result == expected? "Success" : "FAILED") \
<< endl; \
}()
#define TEST_ALL_VARIABLES( expression ) \
TEST( v, expression, true ); \
TEST( u, expression, false ); \
TEST( w, expression, false )
using Fp_info = numeric_limits<double>;
inline auto is_ieee754_nan( double const x )
-> 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<uint64_t const&>( x );
#else
Byte bytes[sizeof(x)];
memcpy( bytes, &x, sizeof( x ) );
uint64_t int_value;
memcpy( &int_value, bytes, sizeof( x ) );
uint64_t const& 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 & exp_mask) == exp_mask and (bits & mantissa_mask) != 0;
}
auto main()
-> int
{
double const v = Fp_info::quiet_NaN();
double const u = 3.14;
double const w = Fp_info::infinity();
cout << boolalpha << left;
cout << "Compiler claims IEEE 754 = " << Fp_info::is_iec559 << endl;
cout << endl;;
TEST_ALL_VARIABLES( std::isnan(value) ); cout << endl;
TEST_ALL_VARIABLES( (fpclassify(value) == FP_NAN) ); cout << endl;
TEST_ALL_VARIABLES( (value != value) ); cout << endl;
TEST_ALL_VARIABLES( (value == Fp_info::quiet_NaN()) ); cout << endl;
TEST_ALL_VARIABLES( (ilogb(value) == FP_ILOGBNAN) ); cout << endl;
TEST_ALL_VARIABLES( isunordered(1.2345, value) ); cout << endl;
TEST_ALL_VARIABLES( is_ieee754_nan( value ) );
}
g++的结果(再次注意，(value == Fp_info::quiet_NaN())的标准行为是它不能作为nan检测器工作，它只是在这里非常实用):
[C:\my\forums\so\282 (detect NaN)] > g++ --version | find "++" g++ (x86_64-win32-sjlj-rev1, Built by MinGW-W64 project) 6.3.0 [C:\my\forums\so\282 (detect NaN)] > g++ foo.cpp && 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)] > g++ foo.cpp -ffast-math && 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)] > _
Visual c++测试结果:
[C:\my\forums\so\282 (detect NaN)] > cl /nologo- 2>&1 | find "++" Microsoft (R) C/C++ Optimizing Compiler Version 19.00.23725 for x86 [C:\my\forums\so\282 (detect NaN)] > cl foo.cpp /Feb && 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)] > cl foo.cpp /Feb /fp:fast && 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)] > _
综上所述，仅使用本测试程序中定义的is_ieee754_nan函数直接测试位级表示，在g++和Visual c++的所有情况下都能可靠地工作。
附录: 在发布上述内容后，我意识到另一种可能的NaN测试，在这里的另一个答案中提到，即((value < 0) == (value >= 0))。这在Visual c++中运行得很好，但在g++的- fast-math选项中失败了。只有直接位模式测试才能可靠地工作。
如果你的编译器支持c99扩展，有一个std::isnan，但我不确定mingw是否支持。
下面是一个小函数，如果你的编译器没有标准函数，它应该可以工作:
bool custom_isnan(double var)
{
volatile double d = var;
return d != d;
}
考虑到(x != x)对于NaN并不总是保证的(比如如果使用- fast-math选项)，我一直在使用:
#define IS_NAN(x) (((x) < 0) == ((x) >= 0))
数字不能同时< 0和>= 0，所以实际上只有当数字既不小于也不大于或等于0时，这个检查才会通过。基本上没有数字，或者NaN。
如果你喜欢，你也可以使用这个:
#define IS_NAN(x) (!((x)<0) && !((x)>=0)
我不确定这是如何受到快速数学的影响，所以你的里程可能会有所不同。
