哈希表

又是一个查找，这个查找思想跟其他的不太一样，是把所有元素都映射到了出来，直接对元素的某一特性的计算，得到它对应的一个存储地址（这个可以存在数组中），到查找的时候，就可以直接对数组名以同样的计算方式算出来存储的地址，可以直接访问该元素。这就形成了一个最有的查找（O(1)）。但是在计算过程中出现重复的结果，这个时候就要根据个人爱好选择或者自己规定一种解决方案，比如说再散列法（即有冲突再以其他不同的计算形式计算出地址，直到不冲突）、链地址法（将冲突的元素链成一条链表）。。。。

 

#define HASHSIZE 32	//32,50,101

//待存入表格数据
char *key_input[] = {
		"auto",   "break",  "case",     "char",   "const",    "continue", "default",  
		"do",
		"double", "else",   "enum",     "extern", "float",    "for",      "goto",     
		"if",
		"int",    "long",   "register", "return", "short",    "signed",   "sizeof",   
		"static",
		"struct", "switch", "typedef",  "union",  "unsigned", "void",     "volatile",
		"while"
};
char *key_help_temp[] = {
		"auto function",
		"break function",
		"case function",
		"char function",
		"const function",
		"continue function",
		"default function",  
		"do function",
		"double function",
		"else function",
		"enum function",
		"extern function",
		"float function",
		"for function",
		"goto function",     
		"if function",
		"int function",
		"long function",
		"register function",
		"return function",
		"short function",
		"signed function",
		"sizeof function",   
		"static function",
		"struct function",
		"switch function",
		"typedef function",
		"union function",
		"unsigned function",
		"void function",
		"volatile function",
		"while function"
};
char key_help[HASHSIZE][100];
static char val_flag[HASHSIZE];//标志已占用存储单元

void ClearFlag()
{
	int i;
	
	for (i = 0;i < HASHSIZE;i++)
	{
		val_flag[i] = (HASHSIZE+1);//清标志位
	}
}

//哈希函数，从数据中抽出某个成员用于哈希值的计算
unsigned int hash(char *s)
{
	unsigned int hashval;
	int i = 0;

	for (hashval = 0; *s != '\0'; s++)
		hashval = *s + 31 * hashval;
	hashval = hashval % HASHSIZE; //计算下标

	while ((val_flag[hashval] != (HASHSIZE+1)) && ((i+hashval)<HASHSIZE))
	{
		i++;
		hashval = (hashval + i)%HASHSIZE;	//冲突处理，存储单元(下标)偏移
	}
	if (i<HASHSIZE)
	{
		printf("\n元素下标[%d]: 冲突次数： < %d >  ",hashval,i);
		val_flag[hashval] = hashval; //表示该单元被占用
		return hashval;
	}
	return -1;
}

//功能：输入一个C语言关键字，快速提示这个关键字的功能及用法
int main(void) 
{
  int i, size, pos;

  size = sizeof(key_input) / sizeof(key_input[0]);//计算关键字数量
  
  //将数据存入哈希表
  ClearFlag();  
  //摸拟关键字输入，并根据hash值存储关键字使用的功能方法到key_help中
  for(i = 0;i < size; i++)
	  strcpy(key_help[hash(key_input[i])],key_help_temp[i]);

  printf("\n**************************************\n");
  ClearFlag(); 
  //摸拟关键字输入，并根据与创建相同的hash值，从key_help中查找对应的功能方法
  for(i = 0; i < size; i++)
  {
    pos = hash(key_input[i]);	
    //printf("%12s  [%d]", key_help[pos], pos);
    printf("%s  ", key_help[pos]);
  }
  printf("\n\n");
  return 0;
}