# Traditional loop
squares = []
for x in range(10):
    squares.append(x ** 2)

# List comprehension — same result, one line
squares = [x ** 2 for x in range(10)]
# [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

evens = [x for x in range(20) if x % 2 == 0]
# [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

words = ["hello", "world", "python", "code"]
long_words = [w.upper() for w in words if len(w) > 4]
# ['HELLO', 'WORLD', 'PYTHON']

# Flatten 2D list
matrix = [[1,2,3],[4,5,6],[7,8,9]]
flat = [n for row in matrix for n in row]
# [1, 2, 3, 4, 5, 6, 7, 8, 9]

# 5×5 multiplication table
table = [[i * j for j in range(1, 6)] for i in range(1, 6)]

# Dict comprehension
word_lengths = {word: len(word) for word in ["apple", "banana", "cherry"]}
# {'apple': 5, 'banana': 6, 'cherry': 6}

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

# Set comprehension — removes duplicates
unique_lengths = {len(w) for w in ["cat", "dog", "bird", "ant", "fish"]}
# {3, 4}

# List comprehension — creates full list in memory immediately
squares_list = [x**2 for x in range(1_000_000)]   # 8 MB memory

# Generator expression — lazy: computes one value at a time
squares_gen = (x**2 for x in range(1_000_000))     # ~100 bytes

# Same iteration interface
total = sum(x**2 for x in range(1_000_000))   # no list ever created

def fibonacci(n):
    a, b = 0, 1
    for _ in range(n):
        yield a           # pauses here, returns a
        a, b = b, a + b   # resumes here on next()

for num in fibonacci(10):
    print(num)    # 0 1 1 2 3 5 8 13 21 34

def chain(*iterables):
    for it in iterables:
        yield from it     # delegates to sub-generator

list(chain([1,2], [3,4], [5,6]))   # [1, 2, 3, 4, 5, 6]

def counter(start=0):
    n = start
    while True:
        received = yield n    # can receive values with send()
        n = (received or n) + 1

gen = counter(10)
next(gen)        # 10 — prime the generator
next(gen)        # 11
gen.send(100)    # 101 — jump to 100, then +1
gen.throw(ValueError, "stop")   # raise in generator
gen.close()      # StopIteration

import itertools

# Infinite counter
for i in itertools.count(start=10, step=2):   # 10, 12, 14, ...
    if i > 20: break

# Repeat value
list(itertools.repeat("x", 4))    # ['x', 'x', 'x', 'x']

# Cycle through iterable
colors = itertools.cycle(["red", "green", "blue"])
[next(colors) for _ in range(6)]  # ['red', 'green', 'blue', 'red', ...]

# Combinations and permutations
list(itertools.combinations("ABC", 2))   # [('A','B'),('A','C'),('B','C')]
list(itertools.permutations("AB", 2))    # [('A','B'),('B','A')]
list(itertools.product("AB", "12"))      # [('A','1'),('A','2'),('B','1'),('B','2')]

# Chaining iterables
list(itertools.chain([1,2], [3,4], [5]))   # [1, 2, 3, 4, 5]

# islice — lazy slice of any iterable
list(itertools.islice(range(100), 5, 15, 2))   # [5, 7, 9, 11, 13]

# groupby — group consecutive identical elements
data = [("a",1),("a",2),("b",3),("b",4),("c",5)]
for key, group in itertools.groupby(data, key=lambda x: x[0]):
    print(key, list(group))