Phase 1 · Day 9 · Learn AI

What is a set?

A set is an unordered collection of unique items. Duplicates are dropped automatically.

Create with curly braces {1, 2, 3} or set(...). An empty set must be set()—{} makes an empty dict.

Creating sets

tags = {"nlp", "cv", "nlp", "rl"}
print(tags)  # duplicates removed

nums = set([3, 1, 4, 1, 5, 9, 3])
print(nums)

empty = set()
print(type(empty), type({}))

Set items must be hashable (immutable): numbers, strings, tuples—not lists or dicts.

Membership and set operations

in is very fast on sets. Use add and discard/remove to update.

Combine sets with union |, intersection &, difference -, and symmetric difference ^.

Add, check, combine

skills = {"python", "git"}
skills.add("sql")
print("git" in skills)

a = {1, 2, 3, 4}
b = {3, 4, 5}

print(a | b)  # union
print(a & b)  # intersection
print(a - b)  # in a but not b
print(a ^ b)  # in one set only

discard(x) removes x if present and does nothing otherwise. remove(x) raises KeyError if missing.

List patterns you’ll reuse

Deduplicate while optionally preserving order. Filter and map with comprehensions.

When order does not matter and you only care about unique values, convert to a set.

Unique while keeping order

items = ["ada", "grace", "ada", "alan", "grace"]
unique = list(dict.fromkeys(items))
print(unique)  # ['ada', 'grace', 'alan']

# when order does not matter:
print(set(items))

Filter and transform

scores = [88, 55, 91, 70, 64]
passed = [s for s in scores if s >= 70]
labels = ["pass" if s >= 70 else "retry" for s in scores]
print(passed)
print(labels)

Dict + set recipes

Group values, count keys, and compare key sets between dictionaries.

These patterns show up in EDA, feature checks, and simple analytics scripts.

Counting and key diffs

words = ["ai", "ml", "ai", "nlp", "ml", "ai"]
counts = {}
for w in words:
    counts[w] = counts.get(w, 0) + 1
print(counts)

old = {"name": "Ada", "role": "eng"}
new = {"name": "Ada", "team": "AI", "role": "eng"}
print(set(new) - set(old))  # {'team'} new keys
print(set(old) & set(new))  # shared keys

Iterating a dict yields keys. set(my_dict) is the set of keys.

Practice

Mini challenge: skill matcher

You have required = {"python", "git", "sql"} and a candidate’s skills as a comma-separated input string. Print matched skills, missing skills, and whether they qualify (no missing skills).

challenge starter

required = {"python", "git", "sql"}
raw = input("Skills (comma-separated): ")

# TODO: build a set of candidate skills (strip + lower)
# print matched, missing, and qualifies True/False

Interactive

Try It in the browser

Create set a = {1,2,3,4,5} and b = {4,5,6,7}. Print union, intersection, and values only in a.

Output

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Errors

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Sets & Collection Patterns