Learn Hash Tables in 13 minutes #️⃣

Summary

This script explains hash tables as a data structure for storing key-value pairs efficiently. It covers how keys are hashed to find their index in the table, using modulus to handle large numbers. Collisions, where different keys hash to the same index, are resolved using linked lists. The script also discusses the impact of table size on collisions and efficiency, with a focus on Java implementation and practical examples.

Takeaways

• 🔑 **Key-Value Pairs**: A hash table is a collection of key-value pairs, where each pair is called an entry.
• 📚 **Data Types**: The key and value can be of any data type, such as integers and strings.
• 🔢 **Hash Function**: The hash function takes a key and converts it into an integer called a hash.
• 🔄 **Collision Handling**: If two keys have the same hash, it results in a collision, which is resolved by chaining.
• 🔄 **Chaining**: Collisions are handled by turning each bucket into a linked list if more than one entry is at the same index.
• 📏 **Index Calculation**: The index for placing an entry is calculated by taking the hash modulo the capacity of the hash table.
• 🔍 **Lookup Efficiency**: Hash tables are efficient for lookups, insertions, and deletions, especially with a low number of collisions.
• 📈 **Dynamic Resizing**: Hash tables can dynamically resize to accommodate more elements, maintaining efficiency.
• 💾 **Memory Usage**: Increasing the size of the hash table reduces collisions but increases memory usage.
• 📊 **Runtime Complexity**: The runtime complexity of a hash table is between O(1) for best case (no collisions) and O(n) for worst case (all entries in one bucket).

Q & A

What is a hash table?

A hash table is a data structure that stores unique keys mapped to values, where each key-value pair is known as an entry.

What are the two pieces of data in each entry of a hash table?

Each entry in a hash table consists of a key and a value, where the key is used to access the corresponding value.

How does a hash table determine the index for each entry?

A hash table uses a hashcode method to convert the key into an integer called a hash. Then, it calculates the index by taking the hash modulo the capacity of the hash table.

What happens when two keys have the same hash value?

When two keys have the same hash value, it's called a collision. To resolve this, each bucket in the hash table can be turned into a linked list to store multiple entries.

How can collisions in a hash table be reduced?

Collisions can be reduced by increasing the size of the hash table, which decreases the likelihood of multiple keys having the same hash value.

What is the most common resolution for collisions in a hash table?

The most common resolution for collisions is chaining, where each bucket is turned into a linked list to store multiple entries that hash to the same index.

How does the size of the hash table affect its efficiency?

A larger hash table size reduces the number of collisions, increasing lookup efficiency. However, it also increases memory usage.

What is the runtime complexity of a hash table in the best and worst cases?

The best-case runtime complexity of a hash table is O(1), when there are no collisions. The worst-case complexity is O(n), when all entries collide and are stored in a single bucket as a linked list.

How does the data type of the key affect the hash code in a hash table?

Different data types have different hash code formulas. For example, the hash code of an integer key is the number itself, while the hash code of a string key is calculated by summing the ASCII values of its characters.

How can you access and display all key-value pairs in a hash table?

You can access values using the get method with a key, and display all pairs by iterating over the keys using an enhanced for loop and printing both the key and the value retrieved by the get method.

What is the purpose of the modulus operator in the context of hash tables?

The modulus operator is used to find the remainder when the hash value is divided by the hash table's capacity, which determines the index or bucket where the entry will be placed.