在双精度数组上使用unordered_map [英] Using unordered_map on array of doubles
问题描述
我的主数据对象是一个长度的二进制数组,取决于我的类的具体实例。我想构造一个非常简单的哈希表来存储/检索这些对象,我们可以假设数字是以没有数字错误的方式生成的。
My main data object is a array of doubles of a length that depends on a specific instantiation of my class. I would like to construct a very simple hash table to store/retrieve these objects, and we can assume that the numbers are generated in a way that is free of numerical error.
int main() {
std::tr1::unordered_map<double*, double*> cache;
double x1[] = { 1.0, 3.14 };
double x2[] = { 1.0, 3.14 };
cache[x1] = x1;
std::cout << "x1: " << cache.count(x1) << std::endl;
std::cout << "x2: " << cache.count(x2) << std::endl;
return 0;
}
上面显然只比较了指针,给出了输出:
The above obviously only compares the pointers, giving the output:
> ./tmp
x1: 1
x2: 0
当我真的想要看到:
> ./tmp
x1: 1
x2: 1
很明显当编译时固定数组的大小时,创建自定义散列和等式函数,但是我不知道如何使定制函数依赖于一个特定的实例化...我在下面创建了一个类,但是我不知道它是有用的,还是可以使用的。
It's pretty clear how to create custom hashing and equality functions when the size of the arrays are fixed at compile time but I do not know how to make custom functions that depend on a specific instantiation... I created a class below, but I'm not sure if it's useful, or how it could be used.
class Hash_double_vec {
public:
int dim;
Hash_double_vec(int d) { dim = d; }
size_t operator()(const double *x) const{
std::tr1::hash<double> hash_fn;
size_t r = hash_fn(x[0]);
for(int i=1;i<dim;i++) r ^= hash_fn(x[i]);
return r;
}
bool operator()(const double *x, const double *y) const{
for(int i=1;i<dim;i++) if (fabs(x[i]-y[i]) > 1e-10) return false;
return true;
}
};
推荐答案
一种方法是创建struct来保存指针到双打的顺序:
One way would be to create struct to hold the pointer to the sequence of doubles:
struct DoubleRegion
{
double* p;
size_t size;
};
bool operator==(DoubleRegion a, DoubleRegion b)
{
return a.size == b.size && memcmp(a.p, b.p, a.size) == 0;
}
size_t hash(DoubleRegion dr)
{
size_t h = 0;
for (double* p = dr.p; p != dr.p + dr.size; ++p)
h ^= hash(*p);
return h;
}
然后使用它:
unordered_map<DoubleRegion, DoubleRegion> cache;
当然这是您的问题,以确保后备内存的生命周期是生命周期的超集的双重地区。
Of course it is your problem to make sure the lifetime of the backing memory is a superset of the lifetime of the DoubleRegion.
旧答案:
如果你不知道直到运行时,键和值将会有多大,请使用std :: vector:
If you don't know until runtime how big the key and value is going to be, use a std::vector:
unordered_map<vector<double>, vector<double>> cache;
如果您在编译时知道可以使用std ::数组大小:
If you know at compile-time how big you can use an std::array:
unordered_map<array<double, N>, array<double, N>> cache;
在这两种情况下,默认散列函数将根据需要按值工作,您不需要定义一个定制的。
In both cases the default hashing function will work by value as you want, and you do not need to define a custom one.
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