Why dictionaries are used in Python - See It in Action

Why dictionaries are used

📖 Scenario: Imagine you run a small library. You want to keep track of books and their authors quickly. You need a way to find the author of any book easily.

🎯 Goal: You will create a dictionary to store book titles and their authors. Then, you will look up authors by book titles to see why dictionaries are useful.

📋 What You'll Learn

Create a dictionary called library with exact book-author pairs

Create a variable called book_to_find with a specific book title

Use dictionary access to get the author of book_to_find

Print the author name

💡 Why This Matters

🌍 Real World

Libraries, stores, and many apps use dictionaries to quickly find information by a unique key like a book title or product code.

💼 Career

Knowing how to use dictionaries helps you work with data efficiently, a key skill in programming jobs like web development, data analysis, and software engineering.

1

Create the library dictionary

Create a dictionary called library with these exact entries: 'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'

Python

# Create the library dictionary with book titles and authors
# Your code here

Hint

Use curly braces {} to create a dictionary. Each entry has a book title as key and author as value.

2

Choose a book to find

Create a variable called book_to_find and set it to the string '1984'

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
# Create the variable book_to_find with the value '1984'
# Your code here

Hint

Use = to assign the string '1984' to the variable book_to_find.

3

Find the author using the dictionary

Create a variable called author and set it to the value from library for the key book_to_find

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
book_to_find = '1984'
# Get the author from the library dictionary using book_to_find
# Your code here

Hint

Use square brackets [] with the dictionary name and key variable to get the value.

4

Print the author

Write a print statement to display the value of the variable author

Python

library = {'The Hobbit': 'J.R.R. Tolkien', '1984': 'George Orwell', 'Pride and Prejudice': 'Jane Austen'}
book_to_find = '1984'
author = library[book_to_find]
# Print the author
# Your code here

Hint

Use print(author) to show the author's name on the screen.

Practice

1. Why do programmers use dictionaries in Python?
my_dict = {'name': 'Alice', 'age': 25}

easy

A. To create loops automatically

B. To store data with unique keys for quick access

C. To perform mathematical calculations faster

D. To store data in a fixed order only

Solution

1. Step 1: Understand dictionary structure

   Dictionaries store data as key-value pairs, where each key is unique.

2. Step 2: Identify the main use

   This structure allows quick access to values using keys, unlike lists which use indexes.

3. Final Answer:

   To store data with unique keys for quick access -> Option B

4. Quick Check:

   Dictionaries = unique keys + fast access [OK]

Hint: Dictionaries use keys to find data fast [OK]

Common Mistakes:

• Thinking dictionaries store data in order only
• Confusing dictionaries with lists for calculations
• Believing dictionaries create loops automatically

2. Which of the following is the correct way to create a dictionary in Python?

easy

A. my_dict = ('key1', 'value1', 'key2', 'value2')

B. my_dict = ['key1', 'value1', 'key2', 'value2']

C. my_dict = {'key1': 'value1', 'key2': 'value2'}

D. my_dict = 'key1': 'value1', 'key2': 'value2'

Solution

1. Step 1: Recognize dictionary syntax

   Dictionaries use curly braces {} with key:value pairs separated by commas.

2. Step 2: Compare options

   my_dict = {'key1': 'value1', 'key2': 'value2'} uses correct syntax with braces and colons; others use lists, tuples, or invalid syntax.

3. Final Answer:

   my_dict = {'key1': 'value1', 'key2': 'value2'} -> Option C

4. Quick Check:

   Curly braces + key:value pairs = dictionary [OK]

Hint: Dictionaries use curly braces and colons [OK]

Common Mistakes:

• Using square brackets instead of curly braces
• Using tuples instead of dictionaries
• Missing curly braces or colons

3. What will be the output of this code?

phone_book = {'Alice': '1234', 'Bob': '5678'}
print(phone_book['Bob'])

medium

A. 5678

B. Bob

C. 1234

D. KeyError

Solution

1. Step 1: Understand dictionary key access

   Accessing phone_book['Bob'] retrieves the value for key 'Bob'.

2. Step 2: Find the value for 'Bob'

   In the dictionary, 'Bob' maps to '5678'.

3. Final Answer:

   5678 -> Option A

4. Quick Check:

   phone_book['Bob'] = '5678' [OK]

Hint: Access value by key inside brackets [OK]

Common Mistakes:

• Confusing keys and values
• Expecting the key name as output
• Mistyping key causing KeyError

4. Find the error in this code that tries to add a new entry to a dictionary:

contacts = {'John': '1111'}
contacts.add('Mary', '2222')

medium

A. Missing parentheses in dictionary creation

B. Dictionary keys cannot be names

C. Keys must be integers, not strings

D. Using .add() method which does not exist for dictionaries

Solution

1. Step 1: Check method used to add items

   Dictionaries do not have an .add() method; items are added by assignment.

2. Step 2: Correct way to add entry

   Use contacts['Mary'] = '2222' to add a new key-value pair.

3. Final Answer:

   Using .add() method which does not exist for dictionaries -> Option D

4. Quick Check:

   Add items by assignment, not .add() [OK]

Hint: Add dict items with assignment, not .add() [OK]

Common Mistakes:

• Trying to use set methods on dictionaries
• Confusing dictionary syntax with list methods
• Assuming keys must be numbers

5. You have a list of students and their scores:

students = [('Anna', 85), ('Ben', 90), ('Anna', 95)]

How can you create a dictionary that stores the highest score for each student?

hard

A. Use a loop to update the dictionary only if the new score is higher

B. Use a dictionary comprehension without conditions

C. Store all scores in a list as dictionary values

D. Use a set instead of a dictionary

Solution

1. Step 1: Understand the problem

   We want one highest score per student, so duplicates must be checked.

2. Step 2: Use a loop to update scores

   Loop through the list, and for each student, update the dictionary only if the new score is higher than the current stored score.

3. Final Answer:

   Use a loop to update the dictionary only if the new score is higher -> Option A

4. Quick Check:

   Update dict with max score per key [OK]

Hint: Update dict values only if new value is higher [OK]

Common Mistakes:

• Using comprehension without checking scores
• Storing multiple scores instead of highest
• Using sets which don't map keys to values