std::numeric_limits::epsilon
static T epsilon(); | ? | (until C++11) |
---|---|---|
static constexpr T epsilon(); | ? | (since C++11) |
返回机器epsilon,即1.0
和下一个可由浮点类型表示的值。T
.只有在下列情况下才有意义std::numeric_limits<T>::is_integer==false
...
返回值
T | std::numeric_limits<T>::epsilon() |
---|---|
/* non-specialized */ | T(); |
bool | false |
char | ?0? |
signed char | ?0? |
unsigned char | ?0? |
wchar_t | ?0? |
char16_t | ?0? |
char32_t | ?0? |
short | ?0? |
unsigned short | ?0? |
int | ?0? |
unsigned int | ?0? |
long | ?0? |
unsigned long | ?0? |
long long | ?0? |
unsigned long long | ?0? |
float | FLT_EPSILON |
double | DBL_EPSILON |
long double | LDBL_EPSILON |
例外
(none) | (until C++11) |
---|---|
noexcept specification: noexcept | (since C++11) |
例
演示如何使用机器epsilon比较浮点值是否相等。
二次
#include <cmath>
#include <limits>
#include <iomanip>
#include <iostream>
#include <type_traits>
#include <algorithm>
template<class T>
typename std::enable_if<!std::numeric_limits<T>::is_integer, bool>::type
almost_equal(T x, T y, int ulp)
{
// the machine epsilon has to be scaled to the magnitude of the values used
// and multiplied by the desired precision in ULPs (units in the last place)
return std::abs(x-y) < std::numeric_limits<T>::epsilon() * std::abs(x+y) * ulp
// unless the result is subnormal
|| std::abs(x-y) < std::numeric_limits<T>::min();
}
int main()
{
double d1 = 0.2;
double d2 = 1 / std::sqrt(5) / std::sqrt(5);
if(d1 == d2)
std::cout << "d1 == d2\n";
else
std::cout << "d1 != d2\n";
if(almost_equal(d1, d2, 2))
std::cout << "d1 almost equals d2\n";
else
std::cout << "d1 does not almost equal d2\n";
}
二次
产出:
二次
d1 != d2
d1 almost equals d2
二次
另见
nextafternexttoward (C++11)(C++11) | next representable floating point value towards the given value (function) |
---|
? cppreference.com
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