lecture.md

Lecture 05: Dictionaries & Sets

Dictionaries and sets allow you to store collections of data where items are key-value pairs or unique values.

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1. Dictionaries

A dictionary is an unordered, mutable collection of key-value pairs. It is defined using curly braces {}.

alien_0 = {"color": "green", "points": 5}

Accessing Values

You access the value by specifying its key inside square brackets:

print(alien_0["color"])   # Output: green

If the key does not exist, Python will raise a KeyError. To avoid this, use the get() method, which returns None or a specified default value if the key is missing:

print(alien_0.get("speed", "slow"))  # Output: slow

Modifying and Adding Key-Value Pairs

# Modifying
alien_0["color"] = "yellow"

# Adding
alien_0["x_position"] = 0

Removing Key-Value Pairs

• del: Permanently deletes a key-value pair.
• pop(): Removes the key and returns its value.

del alien_0["points"]

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2. Sets

A set is an unordered collection of unique elements. Sets do not allow duplicates. They are defined using curly braces {} or the set() constructor.

fruits = {"apple", "banana", "orange", "apple"}
print(fruits)  # Output: {"apple", "banana", "orange"} (duplicates are removed!)

Common Set Operations

• add(): Adds an item.
• remove(): Removes an item (raises error if missing).
• discard(): Removes an item (does not raise error if missing).
• Set operations: Union (|), Intersection (&), Difference (-), Symmetric Difference (^).

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

print(a | b)  # Union: {1, 2, 3, 4, 5}
print(a & b)  # Intersection: {3}
print(a - b)  # Difference: {1, 2}

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🧠 Try It Yourself

1. Create a dictionary representing your favorite programming languages. Key should be the person's name, value should be the language. Use get() to look up a name that isn't in your dictionary.
2. Given a list containing duplicates, convert it to a set to remove duplicates, and then back to a list.