Sets for Membership and Dedup
Test existence in constant time.
Sets for Membership and Dedup is a free Coding Interview Prep lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Coding Interview Prep learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What a Set Buys You
A set stores unique items with no order, and its real power is answering 'is this here?' in roughly constant time. ⚡
Why Not Just a List
Checking membership in a list scans every element, so it is O(n). A set hashes the value and jumps straight to the bucket instead.
Make a Set
Build one with curly braces or the set() call. An empty set must use set() because plain braces make a dictionary.
seen = set()
vowels = {'a', 'e', 'i', 'o', 'u'}The Membership Test
Use the in keyword to ask if a value is present. On a set this is the fast lookup that makes the structure worth reaching for.
if x in seen:
print('already saw it')Adding Elements
Drop a value in with add. Repeating the same value changes nothing, so the set quietly keeps everything unique for you.
seen.add(x)Instant Deduplication
Pass any iterable to set() and duplicates vanish in one pass. It is the shortest way to dedup a contest input.
unique = set(nums)Count Distinct Fast
A common task is 'how many distinct values?'. Wrap the data in a set and take its length in a single line.
distinct_count = len(set(nums))Track What You Have Seen
Build a set as you loop to spot the first repeat. If a value is already in the set, you found a duplicate on the spot.
for v in nums:
if v in seen:
return True
seen.add(v)Set Math for Free
Sets support intersection, union, and difference directly. Comparing two groups becomes one operator instead of nested loops.
common = a & b
only_a = a - bRemove and Discard
Use discard to delete safely even if the value is missing, while remove raises an error on absent keys. Pick the one that matches your intent.
seen.discard(x)Only Hashable Items
Sets need hashable elements, so lists cannot go in but tuples can. Convert a list to a tuple first if you must store it.
grid_seen = set()
grid_seen.add((row, col))Quick Check
You need to know how many different numbers appear in a list.
Recap
A set gives O(1) membership, instant dedup, and easy intersection or difference. Reach for it whenever you ask 'have I seen this?'. 🚀
Frequently asked questions
Is the “Sets for Membership and Dedup” lesson free?
Yes — the full text of “Sets for Membership and Dedup” is free to read here on the web, and the Coding Interview Prep course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Coding Interview Prep course, upgrade to CoddyKit PRO.
What will I learn in “Sets for Membership and Dedup”?
Test existence in constant time. You practise Coding Interview Prep with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Coding Interview Prep?
No prior experience is required. Coding Interview Prep on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Sets for Membership and Dedup” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Coding Interview Prep lesson?
Yes. Every Coding Interview Prep lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Sets for Membership and Dedup
- Dictionaries as Lookup Tables
- Counter and defaultdict in Action
- Group and Bucket with a Map