Question

我被赋予了将静态内存分配的给定哈希表更改为动态的赋值，以便在程序运行时可以分配超过限制的更多内存。我决不是要求解决这个问题，我只是想问一下是否有人知道一个好的起点或者我需要注意的代码的哪些方面因为我有点迷失并且与哈希表混淆。我知道枚举和构造函数需要改变，但我不确定其他的东西。这是给出的代码，并提前感谢任何建议：

#ifndef TABLE1_H
#define TABLE1_H
#include <cstdlib>    // Provides size_t
#include <cassert>  // Provides assert

namespace main_savitch_12A
{
template <class RecordType>
class table
{
public:

enum { CAPACITY = 30 }; 
    // CONSTRUCTOR
    table( );
    // MODIFICATION MEMBER FUNCTIONS
    void insert(const RecordType& entry);
    void remove(int key);
    // CONSTANT MEMBER FUNCTIONS
    bool is_present(int key) const;
    void find(int key, bool& found, RecordType& result) const;
    size_t size( ) const { return used; }
private:
    // MEMBER CONSTANTS -- These are used in the key field of special records.
    enum { NEVER_USED = -1 };
    enum { PREVIOUSLY_USED = -2 };
    // MEMBER VARIABLES
    RecordType data[CAPACITY];
    size_t used;
    // HELPER FUNCTIONS
    size_t hash(int key) const;
    size_t next_index(size_t index) const;
    void find_index(int key, bool& found, size_t& index) const;
    bool never_used(size_t index) const;
    bool is_vacant(size_t index) const;
};

    template <class RecordType>
table<RecordType>::table( )
{
    size_t i;

    used = 0;
    for (i = 0; i < CAPACITY; ++i)
        data[i].key = NEVER_USED;  // Indicates a spot that's never been used.
}

template <class RecordType>
void table<RecordType>::insert(const RecordType& entry)
// Library facilities used: cassert
{
    bool already_present;   // True if entry.key is already in the table
    size_t index;        // data[index] is location for the new entry

    assert(entry.key >= 0);

    // Set index so that data[index] is the spot to place the new entry.
    find_index(entry.key, already_present, index);

    // If the key wasn't already there, then find the location for the new entry.
    if (!already_present)
    {
        assert(size( ) < CAPACITY);
        index = hash(entry.key);
        while (!is_vacant(index))
            index = next_index(index);
        ++used;
    }

    data[index] = entry;
    size_t i;
    for (i=0; i<CAPACITY; i++) cout << data[i].key << ' ';
    cout << endl;
}

template <class RecordType>
void table<RecordType>::remove(int key)
// Library facilities used: cassert
{
    bool found;        // True if key occurs somewhere in the table
    size_t index;   // Spot where data[index].key == key

    assert(key >= 0);

    find_index(key, found, index);
    if (found)
    {   // The key was found, so remove this record and reduce used by 1.
        data[index].key = PREVIOUSLY_USED; // Indicates a spot that's no longer in use.
        --used;
    }
}

template <class RecordType>
bool table<RecordType>::is_present(int key) const
// Library facilities used: assert.h
{
    bool found;
    size_t index;

    assert(key >= 0);

    find_index(key, found, index);
    return found;
}

template <class RecordType>
void table<RecordType>::find(int key, bool& found, RecordType& result) const
// Library facilities used: cassert.h
{
    size_t index;

    assert(key >= 0);

    find_index(key, found, index);
    if (found)
        result = data[index];
}

template <class RecordType>
inline size_t table<RecordType>::hash(int key) const
{
    return (key % CAPACITY);
}

template <class RecordType>
inline size_t table<RecordType>::next_index(size_t index) const
// Library facilities used: cstdlib
{
    return ((index+1) % CAPACITY);
}

template <class RecordType>
void table<RecordType>::find_index(int key, bool& found, size_t& i) const
// Library facilities used: cstdlib
{
size_t count; // Number of entries that have been examined

count = 0;
i = hash(key);
while((count < CAPACITY) && (data[i].key != NEVER_USED) && (data[i].key != key))
{
    ++count;
    i = next_index(i);
}
found = (data[i].key == key);
}

template <class RecordType>
inline bool table<RecordType>::never_used(size_t index) const
{
return (data[index].key == NEVER_USED);
}

template <class RecordType>
inline bool table<RecordType>::is_vacant(size_t index) const
{
return (data[index].key == NEVER_USED);// || (data[index].key == PREVIOUSLY_USED);
}
}

#endif

Answer 1

要考虑几点：

您可以使用vector代替C风格的数组来保存元素，因为它允许动态调整大小。
当您需要扩展表时，您必须重新散列所有现有元素，以将它们放入新容器中的新位置（然后您可以在重新散列完成后与现有容器交换）。 / LI>
您希望能够指定决定何时成长的加载因子。
您需要决定是否要允许容器缩小已分配的空间，再次达到某个阈值。

Answer 2

@Mark B的想法就是答案。

想添加：
建议您的表格大小CAPACITY是素数。用素数修改弱哈希函数hash(key)有助于分散。（好的哈希函数不需要帮助。）

您的成长步骤通常是指数级的，可以构建到查找表中。各种作者建议1.5和1.5之间的比例。 4. e。 G。 Grow2x [] = {0,1,3,7,13,31,61，......（素数低于2的幂）;

假设你每次增长2倍，你的增长负荷是100％。那么你的收缩载荷应该是大约70％（几何平均值为100％/ 2x和100％）。如果您的插入/删除悬停在关键级别，您希望避免增长/缩小。

将静态转换为动态哈希表

2 个答案: