Difference between unordered_map and map - Internal Architecture

So, before we move to the internal architecture lets first take a look from top. Before we move forward just wanted to tell there is ABSOLUTELY NO DIFFERENCE between C++ maps and Java maps.

 

 1. Underlying Data Structure:

• map:
• It is implemented as a self-balancing binary search tree (typically a Red-Black Tree).
• This structure ensures that the elements are always sorted based on the keys.
• As a result, operations like insertion, deletion, and search have a time complexity of O(log n).
• unordered_map:
• It is implemented using a hash table.
• The elements are not stored in any particular order; they are placed in "buckets" based on the hash value of their keys.
• As a result, operations like insertion, deletion, and search typically have an average time complexity of O(1), though in the worst case (due to hash collisions), it can degrade to O(n).

2. Ordering:

• map:
• The elements are stored in a sorted order according to the key.
• If you iterate through a map, you will get the elements in ascending order of their keys.
• unordered_map:
• The elements are not stored in any particular order.
• Iterating through an unordered_map will yield elements in an arbitrary order, which can vary even between different runs of the same program.

3. Use Cases:

• map:
• Useful when you need to maintain a sorted order of keys or need to perform range queries (like finding the first key greater than a given key).
• unordered_map:
• Preferred when you do not need the keys to be in any particular order and performance is a concern, as it is generally faster for insertions, deletions, and lookups due to its average O(1) complexity.

4. Memory Usage:

• map:
• Generally uses more memory per element because of the tree structure which requires storing parent-child pointers.
• unordered_map:
• Generally uses more memory overall due to the hash table's need to handle collisions and maintain buckets.

5. Exception Handling:

• map:
• The at() function throws an out_of_range exception if the key is not found.
• unordered_map:
• Similar to map, the at() function throws an out_of_range exception if the key is not found, but because of the hashing mechanism, there could be differences in performance when exceptions are thrown.

 

 

Internal Architecture 

map (Tree Structure)

Imagine you have a dictionary where all the words are stored in alphabetical order.

• How it works:
• When you add a word, it is automatically placed in the right spot to keep the dictionary alphabetically sorted.
• If you look for a word, you start from the middle, then decide whether to go left or right depending on whether your word comes before or after the middle word (like a decision tree).
• This structure is called a binary search tree.
• Key points:
• Always sorted.
• Slower to insert or find a word compared to unordered storage because it has to find the correct place.
• Good for range-based queries, like finding all words between "apple" and "banana."

unordered_map (Hash Table)

Now, think of a vending machine where items are in different slots based on a code (like A1, B2, C3).

• How it works:
• When you want to store an item (key-value pair), you use a function (called a hash function) to decide which slot it should go into (like a code).
• When you look for an item, you use the same hash function to directly go to that slot.
• This is called a hash table.
• Key points:
• Not sorted; items are just placed in slots.
• Faster on average to insert or find an item because you go straight to its slot.
• Not good for ordering or finding a range of items, just for quick lookups.

Simple Comparison:

• smap is like an alphabetically sorted dictionary. Good for keeping things in order but slower to add or find items.
• unordered_map is like a vending machine with specific codes for each item. It’s fast to find what you want, but there’s no order to the items.

This way, map is organized and sorted, while unordered_map is designed for speed and random access!

 

Few Questions I remember (Across all the rounds)

1. Leetcode Problem : Integer to English Words

2. LeetCode Problem : Robot Collisions

If I remember any other questions, I will add Comments, So You can Check Comment Section of this Blog :)