Basic Data Collections

Introduction to Collections

A single variable holds one value, but a collection holds many. The most common collection is the list (or array), an ordered sequence of values stored under one name.

scores = [90, 85, 70, 100]

Each element occupies a position called its index. Indices almost universally start at 0: scores[0] evaluates to 90, while the final element of a four-item list sits at scores[3] rather than scores[4].

The length of a list is its total number of elements. Lists support adding, removing, updating items, and iterating through them using loops.

Lists vs Dictionaries

A dictionary (or map) stores key-value pairs, allowing fast lookups by name or ID instead of numeric position.

ages = {"Ana": 30, "Ben": 25}

Common Pitfall: Forgetting that indices start at 0. For a list of length n, valid indices range from 0 to n-1. Accessing index n triggers an index-out-of-range error.