C++ 散列搜索函数陷入无休止的“else”和“while”循环

Question

如果生成的要查找的随机数在哈希表数组中不存在，则程序会陷入函数void hashSearch()中的无限循环， 而它应该只是退出循环并输出未找到搜索项。代码中的确切位置是这些输出的位置： cout << "stuck in else loop \n"; 和 cout << "stuck in while loop end \n";。

我用谷歌搜索过，但找不到类似的例子。

#include <iostream>
#include <stdlib.h>     /* srand, rand */
#include <time.h>       /* time */
#include <chrono>
using namespace std;
int arr [1000];
int arr2 [1000];
int randArrayInt, n, randSearchItem, searchInt, address, size2;
void printZeroArr();
void linearSentinelSearch();
void printHashArray();
void hashSearch();
int main ()
{
    srand (time(nullptr));  //initialize random seed:
    n = rand() % 900 + 100; //random integer number from 100 - 1000, length of the array
    //n = rand() % 10; // random number in the range 1-10 for sanity tests, length of the array
    //randSearchItem = rand() % 10 + 1;
    randSearchItem = rand() % 900 + 100; //this is the number to search for
    cout << "Array length is " << n << endl;
    cout << "[";
    for (int i = 0; i <= n; i++)
    {
        randArrayInt = rand() % 900 + 100;
        //randArrayInt = rand() % 10 + 1; // generate random 1-10 number for for sanity tests
        arr[i] = randArrayInt;   // insert into array position the generated random number
        cout<< " " << arr[i];  // print out array element at current loop position
    }
    cout << " ]\n" << endl;
    printZeroArr();
}

void printZeroArr()
{
    size2 = n + 1; //length of hashed array
    cout << "This is the random key to search for in array: " << randSearchItem << endl;
    cout << "This is the size2 length " << size2 << endl;
    cout << "This is the hasharray with zeros" << endl;
    cout << "[";
    for (int i = 0; i <= size2; i++)
    {
        arr2[i] = 0;   // insert into hasharray number 0
        cout<< " " << arr2[i];  // print out hasharray element at current loop position
    }
    cout << " ]\n" << endl;
    linearSentinelSearch();
}

void linearSentinelSearch()
{
    auto start = std::chrono::high_resolution_clock::now();
    arr[n + 1] = randSearchItem;
    //cout << "testing arr[n + 1] is " << arr[n + 1] << endl;
    int i = 0;
    while (arr[i] != randSearchItem) i++;
    if (i == n + 1)
        cout << "Sentinel search did not found the searchitem in random array" << "\n" << endl;
    else
        cout << "Searchitem found in array with linearsearch at position " << i << "\n" << endl;
    auto finish = std::chrono::high_resolution_clock::now();
    chrono::duration<double> elapsed = finish - start;
    cout << "Elapsed time: " << elapsed.count() << " s\n";
    printHashArray();
}

void printHashArray()
{
    //cout << "printing out 'address' value, or the modulo result: " << endl;
    //cout << "[";
    for (int i = 0; i <= n; i++)
    {
        address = arr[i] % size2;
        //cout << " " << address;
        while (arr2[address] != 0)
        {
            if (address == size2 - 1)
            {
                address = 0;
            } else
            {
                address++;
            }
        }
        arr2[address] = arr[i];
    }
    //cout << " ]\n" << endl;
    cout << "This is the hasharray with hashitems" << endl;
    cout << "[";
    for (int i = 0; i <= size2; i++)
    {
        cout << " " << arr2[i];
    }
    cout << " ]\n" << endl; hashSearch();
}

void hashSearch()
{
    auto start = std::chrono::high_resolution_clock::now();
    int searchInt = randSearchItem % size2;
    while ((arr2[searchInt] != 0)  && (arr2[searchInt] != randSearchItem))
    {
        if (searchInt == size2 - 1)
        {
            searchInt = 0;
            cout << "if loop \n";
        }
        else
        {
            searchInt++;
            cout << " stuck in else loop \n";
        }
        cout << " stuck in while loop end \n";
    }
    if (searchInt == 0) {
        cout << "Search item not found using hashSearch" << endl;
    } else {
        cout << "Search item " << randSearchItem << " found using hashSearch at position " << searchInt << " in arr2." << endl;
    }
    auto finish = std::chrono::high_resolution_clock::now();
    chrono::duration<double> elapsed = finish - start;
    cout << "Elapsed time: " << elapsed.count() << " s\n";
}

而它应该只是退出循环并输出未找到搜索项。 搜索cout << " stuck in else loop \n"; 和cout << " stuck in while loop end \n";。

Answer 1

您想在到达数组末尾时停止循环：为此，您将要搜索的项目设置为零：

    if (searchInt == size2 - 1)
    {
        searchInt = 0;
        cout << "if loop \n";
    }

但在循环控制中，您不会对此进行测试。您只测试当前索引处的数组元素是否为零（未找到）或要搜索的项目（找到）：

while ((arr2[searchInt] != 0)  && (arr2[searchInt] != randSearchItem)) ...

您需要额外的测试：

while ((searchInt != 0)  && ...) ...

我花了一段时间才看到您想要编写一个开放地址 hastable，其中零标记未使用的插槽。哈希值只是数字本身。使用零作为空槽的指示符并不理想：您不能存储哈希码以表大小为模后为零的数字。

我还会使用非 void 函数对此进行编码，其中返回值是索引或一些明确的值，意思是“未找到”，可能是 -1。 （或者，如果未找到该项目，您可以返回指向找到的项目的指针或NULL - 毕竟，哈希数组中的索引是哈希表内部的一部分，与调用者无关。）

那么你可以使用提前退货：

int hashSearch(const int *arr2, int size2, int item)
{
    int i = item % size2;

    for (; i < size2; i++) {
        if (arr2[i] == -1) break;            // -1 indicated unused space
        if (arr2[i] == item) return i;       // return index of item
    }

    return -1;     // not found!
}

但是当你的哈希码接近数组大小时，如果没有空间容纳更多元素，你会怎么做？您将需要在最后添加额外的空间，否则您需要环绕。也许这就是您想要通过将索引设置回零来实现的目标。在您的情况下，数组已满，因此没有可以用作循环中断标准的零。您将不得不找到另一个标准。您可以通过使哈希表比条目数大 30% 左右来确保有零。或者您可以尝试检测索引是否已与原始索引转了一圈。

正如在 cmets 中已经向您指出的那样：尝试使用函数参数和局部变量，而不是将所有内容都放入全局空间。此外，函数调用的链接，函数中的最后一件事是调用下一个函数，这很奇怪。将所有顺序调用放到 main 中可能会更好。