Why dictionaries are used in Python

Introduction

Dictionaries help you store and find data quickly by using keys instead of just numbers. They make it easy to organize information like a real-life address book.

When you want to look up a phone number by a person's name.

When you need to store settings with names and values, like 'volume' and 10.

When you want to count how many times each word appears in a text.

When you want to group data by categories, like students by their grade.

When you want to quickly check if something exists using a name or label.

Syntax

Python

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

Keys are unique names used to find values quickly.

Values can be any type: numbers, text, lists, or even other dictionaries.

Examples

A dictionary storing names as keys and phone numbers as values.

Python

phone_book = {'Alice': '123-456', 'Bob': '987-654'}

A dictionary storing settings with names and numbers.

Python

settings = {'volume': 10, 'brightness': 70}

An empty dictionary ready to store key-value pairs.

Python

empty_dict = {}

Sample Program

This program creates a dictionary of contacts and shows how to get and add phone numbers using names as keys.

Python

contacts = {
    'John': '555-1234',
    'Jane': '555-5678',
    'Doe': '555-0000'
}

print("John's number is", contacts['John'])

# Add a new contact
contacts['Alice'] = '555-9999'
print("Alice's number is", contacts['Alice'])

Important Notes

Dictionaries are very fast for looking up data compared to lists.

Keys must be immutable types like strings or numbers, not lists or other dictionaries.

You can change values or add new key-value pairs anytime.

Summary

Dictionaries store data with unique keys for quick access.

They are useful for organizing related information like names and phone numbers.

You can add, change, or find data easily using keys.

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