将静态哈希表转换为动态哈希表

Question

我被指派将一个带有静态内存分配的给定哈希表更改为动态，以便在程序运行时可以分配超过限制的更多内存。我并不是要求一个问题的解决方案，我只是问是否有人知道一个好的起点，或者我需要注意代码的哪些方面，因为我对哈希表有点迷茫和困惑。我知道枚举和构造函数需要更改，但我不太确定其他方面。以下是给出的代码，并提前感谢您的建议：

#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

需要考虑的几点：

• 您可以使用vector而不是C样式的数组来保存元素，因为它允许动态调整大小。
• 当您需要增大表时，您必须对所有现有元素进行重新散列，以便将它们放在新容器中的新位置(当重新散列时，您可以与现有容器交换)。

希望能够指定决定何时增长的加载因子。

• 您需要决定是否允许容器再次以某个阈值收缩分配的空间。

Answer 2

@Mark B想法就是答案。

我想补充一下：

建议您的表大小CAPACITY是一个质数。用质数修改弱哈希函数hash(key)有助于分散。(一个好的散列函数不需要帮助。)

您的增长步数通常是指数级的，并且可以构建到一个查找表中。不同的作者建议的比率在1.5和4之间。例如，Grow2 x[]={ 0，1，3，7，13，31，61，...(素数正好在2的幂之下)；

假设你每次增长2倍，你的增长负载是100%。那么你的收缩负载应该是70%左右( 100%/2x和100%的几何平均值)。如果插入/删除操作在临界值附近徘徊，您希望避免增长/收缩。