Control Flow

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"       # This runs (85 >= 80)
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Grade: {grade}")  # Grade: B

# Values that evaluate to False:
# 0, 0.0, "", None, [], {}, (), set(), False

name = ""
if not name:
    print("Name is empty")  # This runs

items = []
if items:       # False for empty collections
    print("Has items")
else:
    print("No items")

age = 20
status = "adult" if age >= 18 else "minor"
# status = "adult"

def describe(value):
    match value:
        case 0:
            return "Zero"
        case 1 | 2 | 3:
            return "Small number"
        case int(n) if n > 100:
            return "Large number"
        case str(s):
            return f"String: {s}"
        case _:
            return "Something else"

# Iterating over a sequence
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Using range()
for i in range(5):       # 0, 1, 2, 3, 4
    print(i)

for i in range(2, 10, 3):  # 2, 5, 8
    print(i)

# Enumerate — get index and value
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Zip — iterate multiple sequences in parallel
names = ["Alice", "Bob", "Charlie"]
scores = [85, 92, 78]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

count = 0
while count < 5:
    print(count)
    count += 1

# Be careful with infinite loops!
while True:
    response = input("Type 'quit' to exit: ")
    if response == "quit":
        break

# break — exit the loop immediately
for n in range(10):
    if n == 5:
        break
    print(n)            # 0, 1, 2, 3, 4

# continue — skip to next iteration
for n in range(5):
    if n == 2:
        continue
    print(n)            # 0, 1, 3, 4

# for/else — else runs if loop completed normally (no break)
for n in range(5):
    if n == 10:
        break
else:
    print("Loop completed without break")  # This runs

for n in range(5):
    if n == 3:
        break
else:
    print("Not reached")  # Doesn't run — loop was broken

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

# Comprehension — cleaner and faster
squares = [n ** 2 for n in range(10)]

# With condition
evens = [n for n in range(20) if n % 2 == 0]

# Nested loops
pairs = [(x, y) for x in [1, 2] for y in ["a", "b"]]
# [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b')]