Dictionaries

Let’s dive into Dictionaries in Python 🗝️📚 with clear explanations, emojis, and practice questions (with solutions)!

What is a Dictionary? 🤔

A dictionary in Python is a mutable (changeable) data structure that stores data as key-value pairs. Think of it like a real dictionary: you look up a word (key) to get its definition (value)¹².

• Keys: Must be unique and hashable (like strings, numbers, or tuples of immutables).
• Values: Can be any data type (even lists or other dictionaries).
• Syntax: Curly braces {} and colon : separate keys and values.

# Example dictionary
student = {
    "name": "Alice",
    "age": 20,
    "major": "CS"
}

📝 Here, “name”, “age”, and “major” are keys; “Alice”, 20, and “CS” are values.

Key Features of Dictionaries 🏆

• Unordered: Items are not stored in a specific order (though Python 3.7+ preserves insertion order).
• Mutable: You can add, change, or remove items after creation.
• No Duplicate Keys: Each key must be unique. If you assign a value to an existing key, it will overwrite the old value².
• Fast Lookups: Accessing a value by key is very quick.

Creating Dictionaries 🛠️

Empty dictionary:

my_dict = {}
# or
my_dict = dict()

With initial values:

person = {"name": "Bob", "age": 25}

From a list of tuples:

pairs = [("a", 1), ("b", 2)]
my_dict = dict(pairs)

Accessing Items 🔎

• By key:

print(person["name"])  # Output: Bob

If the key doesn’t exist, you get a KeyError.

• Using .get() method:

print(person.get("age"))         # Output: 25
print(person.get("height", 0))   # Output: 0 (default value)

Safer, as it returns None or a default value if the key is missing².

Modifying Dictionaries ✏️

Add or update a key-value pair:

person["city"] = "Delhi"  # Adds a new key
person["age"] = 26        # Updates existing key

Remove an item:

del person["city"]         # Removes key 'city'
removed = person.pop("age") # Removes 'age' and returns its value

Dictionary Methods 🛠️

Looping Through Dictionaries 🔄

Loop through keys:

for key in person:
    print(key)

Loop through values:

for value in person.values():
    print(value)

Loop through key-value pairs:

for key, value in person.items():
    print(f"{key}: {value}")

Practice Questions with Solutions 🏋️‍♂️

1. Create a dictionary with your name, age, and city. Print each value.

my_info = {"name": "Rahul", "age": 21, "city": "Mumbai"}
print(my_info["name"])  # Rahul
print(my_info["age"])   # 21
print(my_info["city"])  # Mumbai

2. Add a new key “country” with value “India” to your dictionary. Then, remove the “city” key.

my_info["country"] = "India"
del my_info["city"]
print(my_info)  # {'name': 'Rahul', 'age': 21, 'country': 'India'}

3. Given the dictionary: fruits = {"apple": 2, "banana": 3, "orange": 1}

Write code to:

• Print all keys
• Print all values
• Print all key-value pairs

fruits = {"apple": 2, "banana": 3, "orange": 1}
print(fruits.keys())   # dict_keys(['apple', 'banana', 'orange'])
print(fruits.values()) # dict_values([2, 3, 1])
print(fruits.items())  # dict_items([('apple', 2), ('banana', 3), ('orange', 1)])

4. Write a function that counts the frequency of each letter in a string and returns a dictionary.

def letter_count(s):
    freq = {}
    for char in s:
        if char in freq:
            freq[char] += 1
        else:
            freq[char] = 1
    return freq

print(letter_count("banana"))  # {'b': 1, 'a': 3, 'n': 2}

5. Check if “apple” is a key in the fruits dictionary.

if "apple" in fruits:
    print("Yes, apple is in the dictionary!")
else:
    print("No, apple is not found.")

Step-by-Step Example: Counting Unique Characters

Problem: Count the number of unique characters in a string using a dictionary.

Step 1: Create an empty dictionary Step 2: Loop through each character in the string Step 3: If character is not in dictionary, add it Step 4: Print the length of the dictionary (number of unique characters)

s = "hello world"
unique_chars = {}
for char in s:
    unique_chars[char] = 1  # Value doesn't matter here
print(len(unique_chars))  # Output: 8 (h, e, l, o,  , w, r, d)

Common Pitfalls ⚠️

• Keys must be hashable: You can’t use lists or other dictionaries as keys.
• Accessing a missing key with [] raises KeyError: Use .get() to avoid errors.
• Duplicate keys are not allowed: The last value assigned to a key will overwrite previous ones.

Dictionary Comprehensions 🧠

You can create dictionaries in a single line using a comprehension:

squares = {x: x*x for x in range(1, 6)}
print(squares)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

Summary Table: Lists vs Dictionaries

Try It Yourself! 🚀

Q: Create a dictionary for three students with their names as keys and their grades as values. Then, print the average grade.

Solution:

grades = {"Anil": 85, "Bina": 92, "Chetan": 78}
average = sum(grades.values()) / len(grades)
print(f"Average grade: {average}")

Dictionaries are super useful for organizing and accessing data quickly by a unique key! Practice using them in your projects and you’ll master them in no time! 😃🔑¹³²