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translation: refine translation of hash_map.md (krahets#1483)
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* Update hash_map.md

* Update hash_map.md

* Update hash_map.md to improve concise
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Enlightenus authored Sep 12, 2024
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# Hash table

A <u>hash table</u>, also known as a <u>hash map</u>, is a data structure that establishes a mapping between keys and values, enabling efficient element retrieval. Specifically, when we input a `key` into the hash table, we can retrive the corresponding `value` in $O(1)$ time complexity.
A <u>hash table</u>, also known as a <u>hash map</u>, is a data structure that establishes a mapping between keys and values, enabling efficient element retrieval. Specifically, when we input a `key` into the hash table, we can retrieve the corresponding `value` in $O(1)$ time complexity.

As shown in the figure below, given $n$ students, each student has two data fields: "Name" and "Student ID". If we want to implement a query function that takes a student ID as input and returns the corresponding name, we can use the hash table shown in the figure below.

![Abstract representation of a hash table](hash_map.assets/hash_table_lookup.png)

In addition to hash tables, arrays and linked lists can also be used to implement query functionality, but the time complexity is different. Their efficiency is compared in the table below:

- **Inserting elements**: Simply append the element to the tail of the array (or linked list). The time complexity of this operation is $O(1)$.
- **Searching for elements**: As the array (or linked list) is unsorted, searching for an element requires traversing through all of the elements. The time complexity of this operation is $O(n)$.
- **Deleting elements**: To remove an element, we first need to locate it. Then, we delete it from the array (or linked list). The time complexity of this operation is $O(n)$.
- **Inserting an element**: Simply append the element to the tail of the array (or linked list). The time complexity of this operation is $O(1)$.
- **Searching for an element**: As the array (or linked list) is unsorted, searching for an element requires traversing through all of the elements. The time complexity of this operation is $O(n)$.
- **Deleting an element**: To remove an element, we first need to locate it. Then, we delete it from the array (or linked list). The time complexity of this operation is $O(n)$.

<p align="center"> Table <id> &nbsp; Comparison of time efficiency for common operations </p>

Expand All @@ -20,7 +20,7 @@ In addition to hash tables, arrays and linked lists can also be used to implemen
| Insert Elements | $O(1)$ | $O(1)$ | $O(1)$ |
| Delete Elements | $O(n)$ | $O(n)$ | $O(1)$ |

It can be seen that **the time complexity for operations (insertion, deletion, searching, and modification) in a hash table is $O(1)$**, which is highly efficient.
As observed, **the time complexity for operations (insertion, deletion, searching, and modification) in a hash table is $O(1)$**, which is highly efficient.

## Common operations of hash table

Expand Down Expand Up @@ -56,7 +56,7 @@ Common operations of a hash table include: initialization, querying, adding key-
unordered_map<int, string> map;

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map[12836] = "Xiao Ha";
map[15937] = "Xiao Luo";
map[16750] = "Xiao Suan";
Expand All @@ -79,7 +79,7 @@ Common operations of a hash table include: initialization, querying, adding key-
Map<Integer, String> map = new HashMap<>();

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map.put(12836, "Xiao Ha");
map.put(15937, "Xiao Luo");
map.put(16750, "Xiao Suan");
Expand All @@ -101,7 +101,7 @@ Common operations of a hash table include: initialization, querying, adding key-
/* Initialize hash table */
Dictionary<int, string> map = new() {
/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
{ 12836, "Xiao Ha" },
{ 15937, "Xiao Luo" },
{ 16750, "Xiao Suan" },
Expand All @@ -125,7 +125,7 @@ Common operations of a hash table include: initialization, querying, adding key-
hmap := make(map[int]string)

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
hmap[12836] = "Xiao Ha"
hmap[15937] = "Xiao Luo"
hmap[16750] = "Xiao Suan"
Expand All @@ -148,7 +148,7 @@ Common operations of a hash table include: initialization, querying, adding key-
var map: [Int: String] = [:]

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map[12836] = "Xiao Ha"
map[15937] = "Xiao Luo"
map[16750] = "Xiao Suan"
Expand Down Expand Up @@ -192,7 +192,7 @@ Common operations of a hash table include: initialization, querying, adding key-
/* Initialize hash table */
const map = new Map<number, string>();
/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map.set(12836, 'Xiao Ha');
map.set(15937, 'Xiao Luo');
map.set(16750, 'Xiao Suan');
Expand Down Expand Up @@ -220,7 +220,7 @@ Common operations of a hash table include: initialization, querying, adding key-
Map<int, String> map = {};

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map[12836] = "Xiao Ha";
map[15937] = "Xiao Luo";
map[16750] = "Xiao Suan";
Expand All @@ -245,7 +245,7 @@ Common operations of a hash table include: initialization, querying, adding key-
let mut map: HashMap<i32, String> = HashMap::new();

/* Add operation */
// Add key-value pair (key, value) to the hash table
// Add key-value pair (key, value) to hash table
map.insert(12836, "Xiao Ha".to_string());
map.insert(15937, "Xiao Luo".to_string());
map.insert(16750, "Xiao Suan".to_string());
Expand Down Expand Up @@ -490,10 +490,10 @@ First, let's consider the simplest case: **implementing a hash table using only

So, how do we locate the corresponding bucket based on the `key`? This is achieved through a <u>hash function</u>. The role of the hash function is to map a larger input space to a smaller output space. In a hash table, the input space consists of all the keys, and the output space consists of all the buckets (array indices). In other words, given a `key`, **we can use the hash function to determine the storage location of the corresponding key-value pair in the array**.

When given a `key`, the calculation process of the hash function consists of the following two steps:
With a given `key`, the calculation of the hash function consists of two steps:

1. Calculate the hash value by using a certain hash algorithm `hash()`.
2. Take the modulus of the hash value with the bucket count (array length) `capacity` to obtain the array `index` corresponding to that key.
2. Take the modulus of the hash value with the bucket count (array length) `capacity` to obtain the array `index` corresponding to the key.

```shell
index = hash(key) % capacity
Expand Down

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