std:map中的浮点键 [英] Floating point keys in std:map
问题描述
以下代码应该在 std :: map
中存在的 3.0
但是由于浮点精度,它不会被找到。
map< double,double& mymap;
mymap [3.0] = 1.0;
double t = 0.0;
for(int i = 0; i <31; i ++)
{
t + = 0.1;
bool contains =(mymap.count(t)> 0);
}
在上面的示例中,包含
将总是 false
。
我目前的解决方法是将 t
乘以0.1,而不是添加0.1,例如:
for(int i = 0; i <31; i ++)
{
t = 0.1 * i;
bool contains =(mymap.count(t)> 0);
}
现在的问题:
如果我使用 double
键,是否有一种方法来引入一个fuzzyCompare到 std :: map
浮点数比较的常见解法通常是类似于 a-b < epsilon
。但我没有看到一个简单的方法来做这个与 std :: map
。
我真的必须在类中封装 double
类型,并覆盖 operator<(...)
#include< functional>
class own_double_less:public std :: binary_function< double,double,bool>
{
public:
own_double_less(double arg_ = 1e-7):epsilon(arg_){}
bool operator()(const double& left,const double& right)const
{
//你可以选择其他方式来决定
//(原始版本是:return left< right;)
return - 右)> epsilon)&& (左<右);
}
double epsilon;
};
//您的地图:
地图< double,double,own_double_less> mymap;
更新:请参阅项目40中的有效STL !
根据建议更新。
The following code is supposed to find the key 3.0
in a std::map
which exists. But due to floating point precision it won't be found.
map<double, double> mymap;
mymap[3.0] = 1.0;
double t = 0.0;
for(int i = 0; i < 31; i++)
{
t += 0.1;
bool contains = (mymap.count(t) > 0);
}
In the above example, contains
will always be false
.
My current workaround is just multiply t
by 0.1 instead of adding 0.1, like this:
for(int i = 0; i < 31; i++)
{
t = 0.1 * i;
bool contains = (mymap.count(t) > 0);
}
Now the question:
Is there a way to introduce a fuzzyCompare to the std::map
if I use double
keys?
The common solution for floating point number comparison is usually something like a-b < epsilon
. But I don't see a straightforward way to do this with std::map
.
Do I really have to encapsulate the double
type in a class and overwrite operator<(...)
to implement this functionality?
You could implement own compare function.
#include <functional>
class own_double_less : public std::binary_function<double,double,bool>
{
public:
own_double_less( double arg_ = 1e-7 ) : epsilon(arg_) {}
bool operator()( const double &left, const double &right ) const
{
// you can choose other way to make decision
// (The original version is: return left < right;)
return (abs(left - right) > epsilon) && (left < right);
}
double epsilon;
};
// your map:
map<double,double,own_double_less> mymap;
Updated: see Item 40 in Effective STL! Updated based on suggestions.
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