April Python Bootcamp Day 10

 

Day 10: Dictionaries in Python – Mastering Key-Value Data

Dictionaries are one of the most powerful and widely used data structures in Python. They allow you to store and manage data in a key-value format, which makes them extremely efficient for searching, mapping, and organizing structured information.

If lists are about ordered collections, dictionaries are about meaningful relationships between data.

---

 What is a Dictionary?

A dictionary is an unordered, mutable collection of key-value pairs.

student = {
    "name": "Piyush",
    "age": 20,
    "course": "Python"
}

Key Points:

• Keys must be unique and immutable (string, number, tuple)
• Values can be of any data type
• Dictionaries are defined using {}

 Why Dictionaries are Important?

• Fast lookup time: O(1) average complexity
• Used in JSON data (APIs)
• Core structure in backend and data science workflows
• Ideal for representing real-world entities

---

 Creating Dictionaries

# Empty dictionary
data = {}

# Using dict()
data = dict(name="Piyush", age=20)

# Nested dictionary
student = {
    "name": "Piyush",
    "marks": {
        "math": 90,
        "science": 85
    }
}

---

 Accessing Values

student = {"name": "Piyush", "age": 20}

print(student["name"])      # Direct access
print(student.get("age"))   # Safe access

Difference:

• [] → Raises error if key not found
• .get() → Returns None (or default value)

---

 Accessing Nested Dictionary

student = {
    "name": "Piyush",
    "marks": {
        "math": 90,
        "science": 85
    }
}

print(student["marks"]["math"])  # 90

Safe Way:

print(student.get("marks", {}).get("math"))

---

 Adding and Updating Values

student = {"name": "Piyush"}

# Add new key
student["age"] = 20

# Update existing key
student["name"] = "Rahul"

---

 Removing Elements

student = {"name": "Piyush", "age": 20}

student.pop("age")
del student["name"]
student.clear()

---

 Dictionary Methods

student = {"name": "Piyush", "age": 20}

student.keys()
student.values()
student.items()

---

 Looping Through Dictionary

student = {"name": "Piyush", "age": 20}

for key in student:
    print(key, student[key])

for key, value in student.items():
    print(key, value)

---

 Real-World Example: Frequency Counter

nums = [1, 2, 2, 3, 3, 3]

freq = {}
for num in nums:
    freq[num] = freq.get(num, 0) + 1

print(freq)

---

Practice Questions

Basic

1. Create a dictionary with 3 key-value pairs and print it
2. Access a value using a key
3. Add a new key-value pair
4. Update an existing value

---

Intermediate

5. Check if a key exists in a dictionary
6. Print all keys and values separately
7. Merge two dictionaries
8. Count frequency of elements in a list using dictionary

---

Advanced

9. Create a nested dictionary for 3 students with marks
10. Sort a dictionary by keys and values
11. Invert a dictionary (swap keys and values)
12. Group elements based on frequency