Linear table lookup
Linear table storage structure
//数据类型
typedef struct
{
int key;
int other;
}ElemType;
//关键字类型
typedef int KeyType;
typedef struct
{
ElemType* elem;//数组基地址
int length;//表长度
int size;//表的容量
}SSTable;
1. Sequential search O(n)
search one by one
int SeqSearch1(const SSTable* st, KeyType k)
{
assert(st);
//数据元素从1号单元开始存放，0号单元留空
for (int i = st->length; i >= 1; i--)//存在条件判断，效率不佳
if (EQ(st->elem[i].key, k))return i;
}
Improvements for removing conditional judgments
int SeqSearch2(const SSTable* st, KeyType k)
{
assert(st);
//在表st中查找关键字为k的记录,若找到，返回记录在数组中的下标，否则返回0
st->elem[0].key = k;//存放监视哨
int i = 0;
for (i = st->length; !EQ(st->elem[i].key, k); i--)//从表尾端开始向前查找
return i;
}
2. Binary search O(logn)
Perform a binary search on an ordered sequence
//二分查找
int BiSearch1(const SSTable* st, KeyType k)//非递归
{
//用二分法在查找表st中查找关键字值为k的记录，若找到则返回在数组中的下标，否则返回为0
int left = 1, right = st->length;
while (left <= right)
{
int mid = (left + right) / 2;
if (EQ(st->elem[mid].key, k))return mid;//查找成功
else if (LT(st->elem[mid].key, k))left = mid + 1;//右半区间
else right = mid - 1;//左半区间
}
return 0;//查找失败
}
int BiSearch2(const SSTable* st, KeyType k,int left ,int right)//递归
{
if (left > right)return -1;
else
{
int mid = (left + right) / 2;
if (EQ(st->elem[mid].key, k))return mid;//查找成功
else if (LT(st->elem[mid].key, k))return BiSearch2(st, k, mid + 1, right);//右半区间
else return BiSearch2(st, k, left, mid - 1);//左半区间
}
}
Disadvantages: It needs to be ordered, and the sorting takes a long time; it is not convenient to insert data.
3. Block search
Divide the sequence into several intervals, the keywords of the intervals are ordered in a certain order, and the data in the block does not need to be ordered.
The so-called quick sequence means that the key values of all records in each block are greater (ascending) or less than (descending) the values of all keys in the previous block. The data elements of the index table respectively record the maximum key and the starting address of each block.
When searching for a keyword, first determine which interval to search for, find a certain block, and search for the keyword sequentially in the block.