3. Data Structures · lesson 8 of 17 · 1 min · January 1, 2024

Sets

A collection of distinct elements.

Another data structure is set, which represents a collection of distinct elements:

sets.py

s = set()
s.add(1)       # s is now { 1 }
s.add(2)       # s is now { 1, 2 }
s.add(2)       # s is still { 1, 2 }
x = len(s)     # equals 2
y = 2 in s     # equals True
z = 3 in s     # equals False

We’ll use sets for two main reasons. The first is that in is a very fast operation on sets. If we have a large collection of items that we want to use for a membership test, a set is more appropriate than a list:

membership.py

stopwords_list = ["a","an","at"] + hundreds_of_other_words + ["yet", "you"]
"zip" in stopwords_list     # False, but have to check every element

stopwords_set = set(stopwords_list)
"zip" in stopwords_set      # very fast to check

The second reason is to find the distinct items in a collection:

distinct.py

item_list = [1, 2, 3, 1, 2, 3]
num_items = len(item_list)                # 6
item_set = set(item_list)                 # {1, 2, 3}
num_distinct_items = len(item_set)        # 3
distinct_item_list = list(item_set)       # [1, 2, 3]

We’ll use sets much less frequently than dicts and lists.