Hashing（ 哈希查找）

利用哈希函數進行高效的key word的檢索

內容：

提供一篇英語文章，搜索在該文章中存在某一單詞的所有句子

代碼如下：

頭文件“prepare.h”——進行程序的準備階段

#include <iostream>
#include <string>

using  namespace std;

const int tablesize = 100000;
int line_num = 0;
int word_num = 0;
string sentence[10000];
struct Node
{
	string data;
	int line;
	Node *next;
	Node()
	{
		line = 0;
		data = "";
		next = NULL;
	}
};

頭文件“Hash_table.h”——哈希類的申明與實現

#include "prepare.h"

class Hash_table
{
public:
	Hash_table();
	~Hash_table();
	int hash(string key);
	void build(string entry, int line);
	int find(string targrt);
private:
	Node *array;
};

Hash_table::Hash_table()
{
	array = new Node[tablesize];
}

Hash_table::~Hash_table()
{
	delete array;
	array = NULL;
}

int Hash_table::hash(string key)
{
	unsigned long hashval = 0;
	int end = key.size();
	for (int i = 0; i < end; i++)
		hashval = 127 * hashval + key[i];
	return hashval%tablesize;
}

void Hash_table::build(string entry, int line)
{
	int position = hash(entry);
	Node *new_one;
	new_one = new Node();
	new_one->data = entry;
	new_one->line = line;
	new_one->next = array[position].next;
	array[position].next = new_one;
}

int Hash_table::find(string target)
{
	int position = hash(target);
	int found = 0, total = 0;
	int line = 0;
	Node *check = array[position].next;
	while (check != NULL && total <= word_num + 1)
	{
		total++;
		if (check->data == target)
		{
			if (line != check->line)
			{
				found = 1;
				line = check->line;
				cout << sentence[line] << endl;
			}
		}
		check = check->next;
	}
	return found;
}

頭文件“read_and_store.h”——進行數據的讀寫過程

#include "Hash_table.h"
#include <fstream>

//逐詞讀入
Hash_table essay;
void read()
{
	ifstream fin("text.txt");     //The essay you want
	string s;
	int count = 0;
	while (fin >> s)
	{
		string tem;
		word_num++;
		int num = s.size();
		if (s[num - 1] == '.')
		{
			sentence[count + 1] += (s + '\n');
			for (int i = 0; i < num - 1; i++)
				tem += s[i];
			essay.build(tem, count + 1);
			count++;
			line_num++;
		}
		else
		{

			if ((s[num - 1] <= 'z' && s[num - 1] >= 'a') || (s[num - 1] <= 'Z' && s[num - 1] >= 'A'))
				tem = s;
			else
			{
				for (int i = 0; i < num - 1; i++)
					tem += s[i];
			}
			//cout<< tem << endl;
			sentence[count + 1] += (s + " ");
			essay.build(tem, count + 1);
		}
	}
	sentence[0] = sentence[line_num];
	line_num += 1;
}

主函數“main.cpp”

#include "read_and_store.h"

void query(string word)
{
	int found = essay.find(word);
	if (found == 0)
		cout << "Word <" << word << "> isn't exist in this essay." << endl << endl;
}

int main()
{
	read();
	string word;
	while (cin >> word)
		query(word);
}