Hash Table

Hash tables (also known as hash maps or dictionaries) are fundamental data structures that provide average

A hash table is a data structure that implements an associative array abstract data type, a structure that can map keys to values. It uses a hash function to compute an index into an array of buckets or slots, from which the desired value can be found.

Real-World Analogy

Think of a hash table like a library catalog system. When you want to find a book, you don’t search through every shelf. Instead, you look up the book’s call number in the catalog, which tells you exactly which shelf and position to find it. The hash function is like the catalog system - it quickly directs you to the right location.

Visual Explanation

Here’s how a hash table works conceptually:

Input Key  →  Hash Function  →  Array Index  →  Value
    ↓              ↓                ↓            ↓
  "apple"    →   hash("apple")  →     3     →  "fruit"
  "banana"   →  hash("banana")  →     7     →  "fruit"
  "cherry"   →  hash("cherry")  →     1     →  "fruit"

The hash function converts keys into array indices, allowing direct access to values without searching through the entire data structure.

When to Use Hash Tables (The Checklist)

Use hash tables when you encounter these problem characteristics:

• Need fast lookups with
  O(1)
  average time complexity
• Working with key-value mappings where you need to associate unique keys with values
• Need to count frequencies of elements in a collection
• Performing set operations like membership testing or deduplication
• Grouping data by common characteristics or properties
• Implementing caching systems to store computed results
• Looking for pairs or complements that satisfy certain conditions

Time & Space Complexity

Common Mistakes

1. Using wrong data structure - Object when Map needed (IE compatibility)
2. Poor key design - Non-primitive keys without proper hashing
3. Not handling missing keys - Accessing undefined properties
4. Infinite loops - Modifying hash table during iteration
5. Hash collisions - Rarely issue with built-in implementations

Next Steps

Ready to practice? Check out our curated problems and code templates to reinforce your understanding.