Video lesson: Hash Tables: From Keys to Buckets

Lesson promise

By the end, the learner should be able to explain the core model for hash tables: from keys to buckets, apply it to a concrete input, and identify when its usual shortcut or guarantee stops applying. This is a recording brief; publish it as a playable lesson after the narration and visual sequence have been produced and reviewed.

Narration draft

A hash table transforms a key into a hash value and uses part of that value to select a bucket or slot. The table then checks key equality because hashes can collide: equal hashes do not prove equal keys. Correctness therefore depends on both a stable hash/equality contract and collision handling.

With separate chaining, each bucket holds a collection of entries. With open addressing, collisions probe other slots according to a rule. As occupancy rises, probe sequences or bucket chains tend to grow, so implementations resize around a chosen load factor. Resizing reassigns entries because bucket positions depend on table capacity.

Expected constant-time lookup relies on assumptions about hash distribution and workload; adversarial keys can create long chains or probe clusters. Mutable keys are dangerous when their hash changes after insertion. Hash tables also do not inherently preserve sorted order and may use more memory than a compact array.

Visual sequence

1. Put the input and assumptions on screen. Ask the learner to predict the next state before revealing it.
2. Animate the representation and show the operation one transition at a time.
3. Pause at the boundary case in the companion article and compare the result with the invariant.
4. End with the exercise prompt: Explain why a map must compare the original key after a hash match. Then describe what goes wrong if a key object changes a field used by its hash after insertion.

Companion material

Use the article, trace, and interactive concept flow as the learner’s written and visual references. The video remains planned until an actual playable media URL and reviewed transcript are available.