More on Dictionaries

More on Dictionaries

Let’s explore more advanced concepts and uses of Python dictionaries 🗝️🚀, with clear explanations, step-by-step examples, and practice questions (with solutions)!

🧠 What Makes Dictionaries Powerful?

• Dictionaries are mappings: Each key maps to a value, allowing fast lookups without knowing the position/index¹².
• Keys must be hashable: This means keys must be immutable (like strings, numbers, or tuples of immutables)².
• Values can be anything: Numbers, strings, lists, even other dictionaries!

🛠️ Advanced Dictionary Features

1. Creating and Modifying Dictionaries

# Creating a dictionary
eng2sp = {"one": "uno", "two": "dos", "three": "tres"}

# Adding or updating a key-value pair
eng2sp["four"] = "cuatro"  # adds new key
eng2sp["one"] = "uno!"     # updates existing key

# Deleting a key
del eng2sp["four"]

# Using pop to remove and get a value
val = eng2sp.pop("two")  # val = "dos"

If you try to access a key that doesn’t exist, you’ll get a KeyError!²

2. Looping Through Dictionaries

d = {"a": 1, "b": 2, "c": 3}

# Loop through keys
for key in d:
    print(key)

# Loop through values
for value in d.values():
    print(value)

# Loop through key-value pairs
for key, value in d.items():
    print(f"{key} => {value}")

.items() gives you both the key and value at once!²

3. Dictionary as a Counter (Histogram)

Dictionaries are perfect for counting things, like letters in a word!

def letter_count(s):
    d = {}
    for char in s:
        d[char] = d.get(char, 0) + 1
    return d

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

The .get(key, default) method avoids KeyError and provides a default value if the key is missing.¹²

4. Reverse Lookup

Find a key from a value (note: values may not be unique!):

def reverse_lookup(d, v):
    for key in d:
        if d[key] == v:
            return key
    raise LookupError("Value does not appear in the dictionary")

This will return the first key it finds with the given value, or raise an error if not found.¹

5. Dictionaries with Lists and Tuples

• Lists as values: Useful for grouping multiple items under a key.
• Tuples as keys: Only if all elements are immutable.

# List as a value
grades = {"Rahul": [90, 92], "Anil": [85, 88]}

# Tuple as a key
directory = {("Sharma", "Anil"): "9876543210"}

You can’t use lists or dictionaries as keys because they are mutable and not hashable!¹²

6. Dictionary Comprehensions

Quickly build a dictionary from a sequence:

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

7. Membership Testing

Check if a key exists:

d = {"a": 1, "b": 2}
print("a" in d)      # True
print("z" not in d)  # True

Membership tests only check keys, not values!¹²

⚠️ Common Dictionary Errors

📝 Practice Questions with Solutions

1. Count Word Frequency in a Sentence

sentence = "to be or not to be"
def word_histogram(s):
    d = {}
    for word in s.split():
        d[word] = d.get(word, 0) + 1
    return d

print(word_histogram(sentence))  # {'to': 2, 'be': 2, 'or': 1, 'not': 1}

2. Find All Keys with a Certain Value

d = {"a": 1, "b": 2, "c": 1, "d": 3}
def keys_with_value(d, v):
    return [k for k, val in d.items() if val == v]

print(keys_with_value(d, 1))  # ['a', 'c']

3. Safe Value Access

d = {"x": 10, "y": 20}
print(d.get("z", "Not found"))  # Not found

4. Using Tuples as Keys

points = {(0,0): "origin", (1,2): "A", (2,1): "B"}
print(points[(1,2)])  # A

5. Practice: Student Grades

Q: Given a dictionary of students and their grades, print the names of all students who scored above 80.

grades = {"Anil": 85, "Bina": 78, "Chetan": 92, "Deepa": 67}
for student, grade in grades.items():
    if grade > 80:
        print(student)
# Output:
# Anil
# Chetan

🏆 Step-by-Step: Inverting a Dictionary

Goal: Swap keys and values (assume all values are unique).

d = {"a": 1, "b": 2, "c": 3}
inverse = {}
for key, value in d.items():
    inverse[value] = key
print(inverse)  # {1: 'a', 2: 'b', 3: 'c'}

If values are not unique, store a list of keys for each value!¹

🧩 Try It Yourself!

Q: Write a function that takes a string and returns a dictionary with the count of each vowel.

def vowel_count(s):
    vowels = "aeiou"
    d = {}
    for char in s.lower():
        if char in vowels:
            d[char] = d.get(char, 0) + 1
    return d

print(vowel_count("Hello World"))  # {'e': 1, 'o': 2}

🎯 Key Takeaways

• Dictionaries are fast, flexible, and essential for real-world Python!
• Keys must be unique and hashable.
• Values can be any type—even lists or other dictionaries!
• Use .get(), .items(), and comprehensions for powerful, safe, and concise code.

Keep experimenting and practicing—dictionaries are everywhere in Python programming! 🐍✨

If you want more challenges or explanations, just ask!