Dictionaries

A dictionary (dict) is a data structure that stores key-value pairs. Let's explore its pratcical usage.

Creating Dictionaries

Using the dict() constructor:

person = dict(name="Alice", age=30, city="New York")
print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}

Using curly braces:

person = {"name": "Alice", "age": 30, "city": "New York"}
print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}

From a sequence of pairs:

data = [('name', 'Alice'), ('age', 30), ('city', 'New York')]
person = dict(data)
print(person)  # Output: {'name': 'Alice', 'age': 30, 'city': 'New York'}

Using dictionary comprehension:

squares = {x: x**2 for x in (1, 3, 5)}
print(squares)  # Output: {1: 1, 3: 9, 5: 25}

Accessing, Adding, Modifying, and Deleting Elements

Accessing values:

person = {"name": "Alice", "age": 30, "city": "New York"}
print(person["name"])  # Output: Alice

Using get() to avoid KeyError:

print(person.get("country", "Unknown"))  # Output: Unknown

Adding new key-value pairs:

person = dict()
person["name"] = "Alice"
person["age"] = 30
print(person)  # Output: {'name': 'Alice', 'age': 30}

Modifying values:

person["age"] = 31
print(person)  # Output: {'name': 'Alice', 'age': 31}

Deleting elements:

• Using pop():

person = {"name": "Alice", "age": 30, "city": "New York"}
age = person.pop("age")
print(age)      # Output: 30
print(person)   # Output: {'name': 'Alice', 'city': 'New York'}

• Using del:

del person["city"]
print(person)   # Output: {'name': 'Alice'}

Setting default values:

person.setdefault("country", "USA")
print(person)   # Output: {'name': 'Alice', 'country': 'USA'}

Sets

A set is an unordered collection of unique elements.

Creating Sets

Using curly braces:

tech_brands = {"Apple", "Google", "Microsoft"}
print(type(tech_brands))  # Output: <class 'set'>
print(tech_brands)        # Output: {'Google', 'Apple', 'Microsoft'}

Using set() constructor:

empty_set = set()
print(empty_set)  # Output: set()

fruits = set(["apple", "banana", "orange"])
print(fruits)     # Output: {'orange', 'banana', 'apple'}

Adding and Removing Elements

Adding elements:

colors = set()
colors.add("red")
colors.add("blue")
print(colors)  # Output: {'red', 'blue'}

Removing elements:

• Using remove():

colors = {"red", "blue", "green"}
colors.remove("green")
print(colors)  # Output: {'red', 'blue'}

• Handling missing keys with discard():

colors.discard("yellow")  # No error, even though 'yellow' doesn't exist
print(colors)             # Output: {'red', 'blue'}

remove() raises a KeyError if the element is not present, while discard() silently does nothing.