Python 24-Day Course - Day 4: Dictionaries

Day 4: Dictionaries

Creating and Accessing Dictionaries

A dictionary is a collection of key-value pairs.

student = {
    "name": "Alice",
    "age": 20,
    "grade": "A"
}

print(student["name"])        # Alice
print(student.get("email", "N/A"))  # N/A (default value)

Key Methods

Method	Description	Example
`keys()`	List of keys	`student.keys()`
`values()`	List of values	`student.values()`
`items()`	Key-value pairs	`student.items()`
`get(k, d)`	Safe access	`student.get("age", 0)`
`update(d)`	Merge	`student.update({"age": 21})`
`pop(k)`	Delete by key	`student.pop("grade")`

Iterating Over Dictionaries

scores = {"math": 90, "english": 85, "science": 92}

for subject, score in scores.items():
    print(f"{subject}: {score} points")

# math: 90 points
# english: 85 points
# science: 92 points

Nested Dictionaries

school = {
    "class_1": {"teacher": "Mr. Kim", "students": 30},
    "class_2": {"teacher": "Ms. Lee", "students": 28},
}

print(school["class_1"]["teacher"])  # Mr. Kim

Today’s Exercises

Write a program that counts word frequencies (input a sentence -> count occurrences of each word).
Create a function that merges two dictionaries, adding values for matching keys.
Find the student with the highest average score from a student grades dictionary.