# Have I Seen This Before? Sets

Count the different members who came to the gym today, and know at each scan whether it's someone's first visit: why counting scans, a list and a dictionary of "true" each fall short, and how your language's set answers "have I seen this before?" in one step.

![Cheat sheet: Have I Seen This Before? Sets](https://fsdtrainingmedia.blob.core.windows.net/public/images/2026/11/9545b801846941df977ffa209480145b.webp)

- Category: Data Structures
- Level: Apprentice
- Tags: data structures, HashSet, HashSet<T>, Set (data-structures), Set (javascript), Set (python)
- Updated: November 3, 2026

Source: https://www.ask-codey.com/cheat-sheets/ds-sets-cheat-sheet