How to use hash search algorithm in C++

How to use the hash search algorithm in C

The hash search algorithm is an efficient search and storage technology that uses keywords through hash The hash function is converted into a fixed-length index, and this index is then used to search in the data structure. In C, we can implement hash search algorithms by using hash containers and hash functions from the standard library. This article explains how to use the hash search algorithm in C and provides specific code examples.

1. Introduce header files and namespaces
   First of all, before using the hash search algorithm in C, you need to introduce the corresponding header files and namespaces.

#include <unordered_set>  // 哈希集合的头文件
#include <unordered_map>  // 哈希映射的头文件

using namespace std;

2. Using hash set
   Hash set (unordered_set) is an unordered and non-repeatable container. We can use hash collections to achieve fast lookup and deduplication.

First, we create a hash collection and add elements to it.

unordered_set<int> hashSet;
hashSet.insert(1);
hashSet.insert(2);
hashSet.insert(3);

Next, we can use the count() function to determine whether an element exists in the hash collection.

bool exist = hashSet.count(2);

We can also use the erase() function to delete elements in the collection.

hashSet.erase(2);

3. Using Hash Map
   Hash map (unordered_map) is a container of key-value pairs. We can use a hash map to quickly find and determine whether a key value exists.

First, we create a hash map and add key-value pairs to it.

unordered_map<string, int> hashMap;
hashMap["apple"] = 3;
hashMap["banana"] = 5;
hashMap["orange"] = 2;

We can use the find() function to find the value in the hash map and determine whether the key-value pair exists.

auto iter = hashMap.find("apple");
if (iter != hashMap.end()) {
    int value = iter->second;
    cout << "apple的值为：" << value << endl;
} else {
    cout << "未找到相关键值对" << endl;
}

We can also use the erase() function to delete key-value pairs in the hash map.

hashMap.erase("banana");

4. Custom hash function
   In the C standard library, the default hash function can meet most needs. But in certain scenarios, we may need to customize the hash function.

We can customize the hash function by overloading the std::hash template.

// 自定义哈希函数
struct MyHash {
    size_t operator()(const string& str) const {
        size_t result = 0;
        for (char c : str) {
            result = result * 31 + c;
        }
        return result;
    }
};

unordered_map<string, int, MyHash> hashMap;

In the above code, we overload the operator() function in the MyHash structure to convert the string into a hash value.

5. Optimizing the efficiency of the hash search algorithm
   In order to further optimize the efficiency of the hash search algorithm, we can adjust the capacity of the hash container or set an appropriate load factor.

// 调整哈希容器的容量
hashMap.resize(100);

// 设置负载因子为0.5
hashMap.max_load_factor(0.5);

The above is an introduction to how to use the hash search algorithm in C and some optimization techniques. I hope it can be helpful to you. In practical applications, the hash search algorithm is an efficient search and storage technology that can greatly improve the execution efficiency of the program. If you are interested in hash search algorithms, you may wish to further research and practice to expand your knowledge and skills.

The above is the detailed content of How to use hash search algorithm in C++. For more information, please follow other related articles on the PHP Chinese website!