String Hashing - 字符串哈希化

以你之姓@ 2022-12-25 13:00 154阅读 0赞

# **String Hashing - 字符串哈希化** #

String Hashing (字符串哈希化) 简单理解就是将一个字符串转换为一个整数。一般情况下，hash 值会比原始值更易储存 (更小) 或比较。

不要将任意字符对应到数字 `0`，一般情况下，将 `a - z` 对应到数字 `1 - 26`，`idx(x) = x - 'a' + 1`。这是 27 进制的表示。

Here is an example of calculating the hash of a `string s`, which contains only lowercase letters. We convert each character of `s` to an integer. Here we use the conversion `a->1, b->2, …, z->26`. Converting `a->0` is not a good idea, because then the hashes of the strings `a, aa, aaa, …` all evaluate to `0`.  
这是一个计算字符串 `s` 的哈希的示例，该字符串仅包含小写字母。我们将 `s` 的每个字符转换为整数。这里我们使用转换 `a->1, b->2, …, z->26`。转换 `a->0` 不是一个好主意，因为字符串 `a, aa, aaa, …` 的哈希值都等于 `0`。

//======================================================================================================================
    // Yongqiang Cheng
    //======================================================================================================================
    
    #include <iostream>
    
    int main() {
        std::string str("abcdefghijklmnopqrstuvwxyz");
    
        std::cout << "The size of str is " << str.length() << " bytes.\n";
    
        for (int i = 0; i < str.length(); ++i) {
            int num = (str[i] - 'a' + 1);
            std::cout << num << " ";
        }
    
        std::cout << std::endl;
        std::cout << "Hello, World!" << std::endl;
        return 0;
    }

/home/yongqiang/CLionProjects/hash_table/cmake-build-debug/hash_table
    The size of str is 26 bytes.
    1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 
    Hello, World!
    
    Process finished with exit code 0

## 1. ##

alphabet ['ælfəbet]：n. 字母表

## References ##

[http://www.partow.net/programming/hashfunctions/][http_www.partow.net_programming_hashfunctions]  
[https://cp-algorithms.com/string/string-hashing.html][https_cp-algorithms.com_string_string-hashing.html]  
[http://pages.cs.wisc.edu/~siff/CS367/Notes/hash.html][http_pages.cs.wisc.edu_siff_CS367_Notes_hash.html]  
[http://www.cse.yorku.ca/~oz/hash.html][http_www.cse.yorku.ca_oz_hash.html]  
[http://cseweb.ucsd.edu/~kube/cls/100/Lectures/lec16/lec16-15.html][http_cseweb.ucsd.edu_kube_cls_100_Lectures_lec16_lec16-15.html]

[http_www.partow.net_programming_hashfunctions]: http://www.partow.net/programming/hashfunctions/
[https_cp-algorithms.com_string_string-hashing.html]: https://cp-algorithms.com/string/string-hashing.html
[http_pages.cs.wisc.edu_siff_CS367_Notes_hash.html]: http://pages.cs.wisc.edu/~siff/CS367/Notes/hash.html
[http_www.cse.yorku.ca_oz_hash.html]: http://www.cse.yorku.ca/~oz/hash.html
[http_cseweb.ucsd.edu_kube_cls_100_Lectures_lec16_lec16-15.html]: http://cseweb.ucsd.edu/~kube/cls/100/Lectures/lec16/lec16-15.html