March 29

... start recording

Questions about anything?

Context - where are we and what's next ?

... check out and discuss the wiki page .

• no class next Monday : this is the last week of the 1st seven
• time for SEPC on Thursday ?

This week - hash tables

• I'd like you all to implement a hash table.
• We'll start today in class, and continue on Thursday.
• ... I'm considering contest with a performance test ... we'll see.

Here are some resources :

• wikipedia: hash table
• A Hash Table in Python - guru99
• runestone pythonds Hashing
• How to implement a hash table in C - Ben Hoyt, March 2021 (!)
• C/Hash Tables - James Aspnes (Yale)

This is an important data structure, a staple for fast lookup. There are many variations with various tradeoffs for different situations. Your goal is to understand the basic idea and to to explore it a bit with some code.

Key ideas to discuss before we start working on this :

• hash function
• collisions
  • exterior (additional data structure outside main array , perhaps linked list)
  • open addressing (everything in array bins, if collision try another address)
• dynamic resizing (increase array size as needed - leave this out for your 1st code)

I have some code in code/hash_table to get us going.

I've also uploaded a classic word_count.py hash table task that we may discuss.

We didn't do breakout rooms. Instead I started to write a kludgy hash table implementation, but didn't get very far. We did talk a little about lists within lists as a way to handle collisions, putting all (key,value) pairs with the same hash value into the same bucket (which is a list). But as those buckets get bigger, the performance of the hash table degrades.

The assignment for now is to read about hash tables so that we can come back on Thursday and work on some code together.