Lesson 3 — Hash map

Hash map: look things up by key

A hash map stores key → value pairs and finds a value by its key almost instantly, no matter how many pairs there are (O(1) on average). In Python it is the dict. d.get(key, 0) returns the value for key, or 0 if the key is missing — perfect for counting.

freq = {}
freq["cat"] = freq.get("cat", 0) + 1   # {'cat': 1}

Why it matters: counting words, caching results and indexing users by ID — dicts are the most used data structure in Python.