Control Flow — Python Execution Control

5.0 What This Teaches

• if / elif / else and truthiness
• for loops with range, enumerate, and zip
• while loops
• break, continue, and the for/else pattern
• match statements (Python 3.10+)
• Conditional expressions (ternary)

5.1 if / elif / else

score = 78

if score >= 90:
    print("A")
elif score >= 80:
    print("B")
elif score >= 70:
    print("C")
else:
    print("Below C")   # prints: Below C

5.2 Truthiness

items = []
if not items:
    print("list is empty")   # idiomatic; no need for len(items) == 0

name = ""
if name:
    print(f"Hello, {name}")
else:
    print("no name given")

count = 42
if count:   # truthy (non-zero)
    print("there are items")

5.3 for Loops

# range(stop), range(start, stop), range(start, stop, step)
for i in range(5):
    print(i, end=" ")   # 0 1 2 3 4

# enumerate gives (index, value) pairs
fruits = ["apple", "banana", "cherry"]
for i, fruit in enumerate(fruits):
    print(f"{i}: {fruit}")

# zip iterates two sequences in parallel
names = ["Alice", "Bob"]
scores = [95, 87]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

5.4 while Loops

n = 1
while n < 100:
    n *= 2
print(n)   # 128

# while True with break for interactive loops
while True:
    response = input("Enter 'quit' to exit: ")
    if response == "quit":
        break
    print(f"You said: {response}")

5.5 break, continue, and for/else

# break exits the loop
for i in range(10):
    if i == 5:
        break
    print(i, end=" ")   # 0 1 2 3 4

# continue skips the rest of the current iteration
for i in range(10):
    if i % 2 == 0:
        continue
    print(i, end=" ")   # 1 3 5 7 9

# for/else: else runs only if loop completed without break
def find_prime(n):
    for divisor in range(2, n):
        if n % divisor == 0:
            print(f"{n} is not prime")
            break
    else:
        print(f"{n} is prime")

5.6 match Statement (Python 3.10+)

command = "quit"

match command:
    case "quit":
        print("Exiting.")
    case "help":
        print("Available commands: quit, help, list")
    case "list":
        print("Nothing in the list.")
    case _:
        print(f"Unknown command: {command}")

5.7 Conditional Expression

x = 7
label = "even" if x % 2 == 0 else "odd"
print(label)   # odd

# Inline conditional
abs_val = x if x >= 0 else -x

5.8 Example - All Together

# Control Flow - fizzbuzz and while convergence.

for i in range(1, 21):
    if i % 15 == 0:
        print("FizzBuzz", end=" ")
    elif i % 3 == 0:
        print("Fizz", end=" ")
    elif i % 5 == 0:
        print("Buzz", end=" ")
    else:
        print(i, end=" ")
print()

# Collatz conjecture: reach 1 from any positive integer
n = 27
steps = 0
while n != 1:
    n = n // 2 if n % 2 == 0 else 3 * n + 1
    steps += 1
print(f"27 reached 1 in {steps} steps")

5.9 Exercise

5.10 Common Mistakes

if x > 0    # SyntaxError: expected ':'
    print(x)

if x = 5:   # SyntaxError - = is assignment, not comparison
    ...
if x == 5:  # correct

5.11 Key Terms