Python 3 — Dictionaries & Sets

# Creating a dictionary
student = {
    "name": "Balaji",
    "age": 25,
    "grade": "A",
    "subjects": ["Math", "Python", "DSA"]
}

# Accessing by key
print(student["name"])       # Balaji
print(student["age"])        # 25
print(student["subjects"])   # ['Math', 'Python', 'DSA']

# .get() — safe access (returns None if key missing)
print(student.get("email"))         # None
print(student.get("email", "N/A"))  # N/A (default value)

# Check if key exists
print("grade" in student)    # True
print("phone" in student)    # False

profile = {"name": "Alice", "age": 30}

# Add new key-value
profile["email"] = "alice@example.com"
profile["city"] = "Hyderabad"
print(profile)

# Update existing value
profile["age"] = 31
print(profile["age"])  # 31

# Update multiple keys at once
profile.update({"age": 32, "phone": "9876543210"})
print(profile)

# Remove keys
del profile["phone"]         # Remove by key
removed = profile.pop("city") # Remove & return
print(f"Removed: {removed}")

# Clear all
temp = {"a": 1, "b": 2}
temp.clear()
print(temp)  # {}

inventory = {
    "apples": 50,
    "bananas": 30,
    "oranges": 75,
    "mangoes": 20
}

# Iterate keys (default)
for fruit in inventory:
    print(fruit)

# Iterate keys explicitly
for key in inventory.keys():
    print(key)

# Iterate values
for quantity in inventory.values():
    print(quantity)

# Iterate key-value pairs (most useful!)
for fruit, qty in inventory.items():
    status = "✅ In Stock" if qty > 25 else "⚠️ Low Stock"
    print(f"{fruit}: {qty} units — {status}")

scores = {"Alice": 92, "Bob": 78}

# setdefault — adds only if key doesn't exist
scores.setdefault("Charlie", 85)
scores.setdefault("Alice", 0)  # Alice already exists, not changed
print(scores)  # {'Alice': 92, 'Bob': 78, 'Charlie': 85}

# Merge dicts (Python 3.9+)
extra = {"Diana": 95, "Eve": 88}
all_scores = scores | extra
print(all_scores)

# Count word frequency
text = "the cat sat on the mat the cat"
word_freq = {}
for word in text.split():
    word_freq[word] = word_freq.get(word, 0) + 1
print(word_freq)

company = {
    "name": "TechCorp",
    "employees": {
        "E001": {"name": "Alice", "role": "Developer", "salary": 80000},
        "E002": {"name": "Bob",   "role": "Designer",  "salary": 70000},
        "E003": {"name": "Carol", "role": "Manager",   "salary": 90000}
    }
}

# Access nested values
print(company["name"])                         # TechCorp
print(company["employees"]["E001"]["name"])    # Alice
print(company["employees"]["E002"]["salary"])  # 70000

# Iterate nested dict
for emp_id, info in company["employees"].items():
    print(f"{emp_id}: {info['name']} ({info['role']}) — ₹{info['salary']:,}")

# Creating sets
fruits = {"apple", "banana", "cherry", "apple", "banana"}
print(fruits)  # Only unique: {'apple', 'banana', 'cherry'}

# Add and remove
fruits.add("mango")
fruits.discard("cherry")  # No error if not found
print(fruits)

# Fast membership test
print("apple" in fruits)   # True (faster than list!)
print("grape" in fruits)   # False

# Set operations
a = {1, 2, 3, 4, 5}
b = {4, 5, 6, 7, 8}

print(a | b)   # Union: {1,2,3,4,5,6,7,8}
print(a & b)   # Intersection: {4,5}
print(a - b)   # Difference: {1,2,3}
print(a ^ b)   # Symmetric diff: {1,2,3,6,7,8}

# Basic dict comprehension: {key: value for item in iterable}
squares = {n: n**2 for n in range(1, 6)}
print(squares)  # {1: 1, 2: 4, 3: 9, 4: 16, 5: 25}

# With condition: word → length (only long words)
words = ["Python", "is", "an", "amazing", "language"]
long_words = {w: len(w) for w in words if len(w) > 3}
print(long_words)  # {'Python': 6, 'amazing': 7, 'language': 8}

# Invert a dictionary (swap keys and values)
original = {"a": 1, "b": 2, "c": 3}
inverted = {v: k for k, v in original.items()}
print(inverted)   # {1: 'a', 2: 'b', 3: 'c'}