Python Lists & Dicts — The Two Collections You Can't Live Without

# Lists: ordered, indexed by number
scores = [87, 92, 76, 95]
print(scores[0])            # 87
print(scores[-1])           # 95 (last item)
scores.append(88)           # add to the end
print(len(scores))          # 5

# Dicts: accessed by key
student = {
    "name": "Ava",
    "grade": 8,
    "subjects": ["math", "art"],
}
print(student["name"])      # Ava
student["age"] = 13         # add a new key
print(student.get("age", 0))# safe access with default

# Without comprehension
doubled = []
for n in [1, 2, 3, 4]:
    doubled.append(n * 2)

# With comprehension — one line
doubled = [n * 2 for n in [1, 2, 3, 4]]

# With a filter
evens = [n for n in range(10) if n % 2 == 0]

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

# Real mini-project: word frequency counter
text = "the cat sat on the mat and the dog sat too"
words = text.split()

freq = {}
for word in words:
    freq[word] = freq.get(word, 0) + 1

# Sort by count, descending
top = sorted(freq.items(), key=lambda pair: pair[1], reverse=True)
print(top[:3])  # [('the', 3), ('sat', 2), ('cat', 1)]