Create Hash-table.md

Author: AmrutaJayanti

dsa/intermediate/Hash-table.md

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+# Hash Table
+
+A hash table (or hash map) is a fundamental data structure in computer science, used for fast data retrieval. Here’s an in-depth look at hash tables:
+
+## Overview
+A hash table is a collection of key-value pairs, where each key is mapped to a value. The key is transformed into an index in an array through a process called hashing. This index is used to quickly locate the value associated with the key.
+
+## Key Concepts
+1. *Hash Function:* This function takes an input (the key) and returns an integer (the hash code), which is used as an index to store the corresponding value in the array. A good hash function distributes keys uniformly across the array.
+2. *Buckets:* Each index in the array is referred to as a bucket. In cases where multiple keys hash to the same index, these keys are stored in the same bucket.
+3. *Collision Handling:* Collisions occur when multiple keys hash to the same index. Common collision handling techniques include:
+      - Chaining: Each bucket points to a linked list of entries that map to the same index.
+      - Open Addressing: When a collision occurs, the algorithm searches for the next open bucket using a probing sequence (linear probing, quadratic probing, or double hashing).
+
+## Operations
+**Insertion:**
+
+Compute the hash code of the key using the hash function.
+Map the hash code to an index in the array.
+Store the key-value pair in the corresponding bucket. Handle collisions appropriately.
+
+**Search:**
+
+Compute the hash code of the key.
+Map the hash code to an index.
+Check the corresponding bucket. If using chaining, search the linked list. If using open addressing, follow the probing sequence until the key is found or an empty bucket is reached.
+
+**Deletion:**
+
+Compute the hash code of the key.
+Map the hash code to an index.
+Locate the key in the bucket and remove the key-value pair. Adjust the structure to maintain efficiency (e.g., rehash elements if necessary).
+
+## Complexity
+Average Time Complexity: O(1) for insert, delete, and search operations in a well-implemented hash table with a good hash function and a load factor that avoids excessive collisions.
+
+Worst-Case Time Complexity: O(n) for insert, delete, and search operations if all keys hash to the same index (highly unlikely with a good hash function).
+
+## Load Factor
+The load factor (α) is the ratio of the number of entries to the number of buckets in the hash table. A high load factor can lead to more collisions, affecting performance. A common practice is to resize the hash table (rehash) when the load factor exceeds a certain threshold.
+
+
+​Load Factor = No. of entries /  No. of Buckets
+ 
+
+**Rehashing**
+
+When the load factor exceeds a threshold, the hash table is resized (usually doubled) and all existing keys are rehashed to the new array. This operation ensures that the hash table maintains efficient performance.
+
+## Practical Considerations
+*Choosing a Hash Function:* A good hash function minimizes collisions and uniformly distributes keys. Common hash functions include division-remainder, multiplication, and universal hashing.
+
+*Dynamic Resizing:* To maintain efficient operations, hash tables are dynamically resized (rehashing) when the load factor becomes too high.
+
+*Memory Usage:* Hash tables can be memory-inefficient if they are sparsely populated. A balance between memory usage and performance needs to be struck.
+
+## Applications
+
+1. Database Indexing: Hash tables are used for indexing databases to allow quick retrieval of records.
+2. Caches: Hash tables underpin the implementation of caches.
+3. Symbol Tables in Compilers: Used for fast lookup of identifiers (variables, functions, etc.) during compilation.
+4. Sets: Hash tables can be used to implement set data structures for efficient membership checking.
+
+## Implementation in CPP
+
+```cpp
+#include <iostream>
+#include <vector>
+#include <list>
+#include <string>
+
+class HashTable {
+private:
+    int size;
+    std::vector<std::list<std::pair<std::string, std::string>>> table;
+
+    int hashFunction(const std::string& key) const {
+        std::hash<std::string> hashFunc;
+        return hashFunc(key) % size;
+    }
+
+public:
+    HashTable(int s) : size(s), table(s) {}
+
+    void insert(const std::string& key, const std::string& value) {
+        int index = hashFunction(key);
+        for (auto& kvp : table[index]) {
+            if (kvp.first == key) {
+                kvp.second = value;
+                return;
+            }
+        }
+        table[index].emplace_back(key, value);
+    }
+
+    std::string search(const std::string& key) const {
+        int index = hashFunction(key);
+        for (const auto& kvp : table[index]) {
+            if (kvp.first == key) {
+                return kvp.second;
+            }
+        }
+        return "Not Found";
+    }
+
+    bool remove(const std::string& key) {
+        int index = hashFunction(key);
+        for (auto it = table[index].begin(); it != table[index].end(); ++it) {
+            if (it->first == key) {
+                table[index].erase(it);
+                return true;
+            }
+        }
+        return false;
+    }
+};
+
+int main() {
+    HashTable ht(10);
+    ht.insert("key1", "value1");
+    ht.insert("key2", "value2");
+
+    std::cout << "Search key1: " << ht.search("key1") << std::endl;  // Output: value1
+    std::cout << "Search key2: " << ht.search("key2") << std::endl;  // Output: value2
+
+    ht.remove("key1");
+    std::cout << "Search key1: " << ht.search("key1") << std::endl;  // Output: Not Found
+
+    return 0;
+}
+```