有没有一种方法来得到一个与epsilon的float的哈希码？ [英] Is there a way to get a hashcode of a float with epsilon?

问题描述

众所周知，用==比较浮点数通常是一个错误。在我写的一个3D向量类（带有浮点部分X，Y，Z）中，如果两个向量的距离被认为是零，那么两个向量被认为是相等的。

 < （$ obj == null）{
 return false; code> public override bool Equals（object obj）
 {
 if（obj == null） $（
）
 
 if（GetType（）！= obj.GetType（））{
 return false; 
} 
 
 float d = DistSq（（Vec）obj）; 
 
 return IsConsideredZero（d）; 
} 
 
 public float DistSq（Vec p）
 {
 Vec d = this  -  p; 
 return d.LengthSq（）; 
 
 
 public float LengthSq（）
 {
 return X * X + Y * Y + Z * Z; 
} 
 
 private const float VEC_COMPARE_EPSILON_ABS = 1E-05f; 
 public static bool IsConsideredZero（float f）
 {
 return Math.Abs​​（f）< VEC_COMPARE_EPSILON_ABS; 
 
 $ / code> 

到目前为止，一切正常。但是，现在我想要得到一个向量的哈希码。我可以看到像 hash =（int）X ^（int）Y ^（int）Z 这样的东西肯定会失败。

最好的办法是：
$ b $ pre $ public override int GetHashCode（）
{
返回0;
}


当然，这样做很糟糕。有什么办法可以得到一个合理的哈希码？ NaNs和其他特殊的价值是可能的，但不太可能，万一​​这一点很重要。

解决方案

假设你想拥有正常的hashcode / equal属性：

$ ul
如果X = Y和Y = Z那么X = Z（传递性）

如果X = Y，那么Y = X（交换性）
X对于所有的X（反射性）

第一个规则是问题 - 因为如果每个值被认为与下一个更大的可表示数字相等，那么最后所有的数字都是相等的。例如，假设一个数字与另一个数字相等，它们在0.1之内：

<0>等于0.08
0.08等于0.16
0.16等于0.24

=> 0等于传统性规则$ 0.16，传递性规则
=> 0等于0.24

（等）

如果你忽略了传递规则，那么你仍然（可能）想要相等的值有相同的hashcode。这有效地强制了传递规则 - 在上面的例子中，0和0.08必须具有相同的哈希码，0和0.16也是一样。因此0和0.16必须具有相同的哈希码，等等。因此，你可以没有有用的哈希码 - 它必须是一个常量。

It is well known that comparing floats by == is usually a mistake. In a 3D-vector class (with float components X, Y, Z) i wrote, two vectors are considered equal if their distance is considered zero.

    public override bool Equals(object obj)
    {
        if (obj == null) {
            return false;
        }

        if (GetType () != obj.GetType ()) {
            return false;
        }

        float d = DistSq ((Vec) obj);

        return IsConsideredZero (d);
    }

    public float DistSq(Vec p)
    {
        Vec d = this - p;
        return d.LengthSq ();
    }

    public float LengthSq()
    {
        return X * X + Y * Y + Z * Z;
    }

    private const float VEC_COMPARE_EPSILON_ABS = 1E-05f;
    public static bool IsConsideredZero(float f)
    {
        return Math.Abs (f) < VEC_COMPARE_EPSILON_ABS;
    }

So far, everything worked fine. However, now i'd like to get a hashcode of the vector. I can see that something like hash = (int)X^(int)Y^(int)Z is bound to fail.

The best i could come up with was:

    public override int GetHashCode()
    {
        return 0;
    }

This, of course, kind of sucks. Is there any way to get a reasonable hashcode? NaNs and other special values are possible, but unlikely, in case that is important.

解决方案

It's impossible assuming you want to have the normal hashcode/equality properties:

• If X = Y and Y = Z then X = Z (transitivity)
• If X = Y then Y = X (commutivity)
• X = X for all X (reflexivity)

The first rule is the problem - because if each value is deemed "equal" to the next greater representable number, you end up with all numbers being equal. For instance, suppose a number is deemed equal to another they're within 0.1:

0 equals 0.08 0.08 equals 0.16 0.16 equals 0.24

=> 0 equals 0.16 by the transitivity rule => 0 equals 0.24 by the transitivity rule

(etc)

If you ignore the transitivity rule, then you still (presumably) want "equal" values to have equal hashcodes. This effectively enforces the transitivity rule - in the above example, 0 and 0.08 have to have equal hashcodes, as do 0 and 0.16. Therefore 0 and 0.16 have to have equal hashcodes, and so on. Therefore you can have no useful hashcode - it has to be a constant.

这篇关于有没有一种方法来得到一个与epsilon的float的哈希码？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！