Control Flow

Loop Patterns

Common Iteration Techniques

A countdown timer needs to go 10, 9, 8... 1, 0. A report processes every 5th record for sampling. A game loop runs until the player quits. Each scenario uses a different loop pattern.

Count up (1 to 10)

The most basic loop pattern - counting from start to end.

count_up.py 
start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")


start = 
end = 

print(f"Counting from {start} to {end}:")
for i in range(start, end + 1):
    print(i)

# Practical: sum numbers in range
total = sum(range(1, 101))
print(f"Sum of 1-100: {total}")

# Or with a loop
total = 0
for i in range(1, 101):
    total += i
print(f"Sum (loop): {total}")

Python's range() makes counting loops clean and readable.

range `range(stop)`, `range(start, stop)`, `range(start, stop, step)`

Count down (countdown timer)

Looping in reverse order - start high, go low.

count_down.py 
start = 

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)


start = 

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)


start = 

print(f"Countdown from {start}:")
for i in range(start, 0, -1):
    print(i)
print("Liftoff!")

# Practical: reverse list traversal
arr = [10, 20, 30, 40, 50]
print("\nArray in reverse:")
for i in range(len(arr) - 1, -1, -1):
    print(f"Index {i}: {arr[i]}")

# Pythonic way: reversed()
print("\nUsing reversed():")
for item in reversed(arr):
    print(item)

# Or slice
print("\nUsing slice:")
for item in arr[::-1]:
    print(item)

range(start, end, step) with negative step counts backwards.

Step by N (every nth element)

Skip elements by using a step value.

step_by_n.py 
step = 

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)

step = 

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)

step = 

# Count by step
print(f"Counting by {step}:")
for i in range(0, 51, step):
    print(i)

# Practical: process every other element
names = ["Alice", "Bob", "Charlie", "Diana", "Eve", "Frank"]
print("\nEvery other name:")
for i in range(0, len(names), 2):
    print(names[i])

# Pythonic: slice with step
print("\nUsing slice:")
for name in names[::2]:
    print(name)

# Odd numbers only
print("\nOdd numbers 1-20:")
odds = list(range(1, 21, 2))
print(odds)

The third argument to range() is the step size.

While with condition

Loop until some condition becomes false.

while_condition.py 
# Guessing game simulation
secret = 
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")


# Guessing game simulation
secret = 
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")


# Guessing game simulation
secret = 
guess = 1
attempts = 0

while guess != secret:
    print(f"Trying {guess}... wrong!")
    guess += 1
    attempts += 1

print(f"Found {secret} in {attempts} attempts!")

# Practical: process until sentinel value
data = [10, 20, 30, -1, 40, 50]  # -1 is sentinel
i = 0
total = 0

print("\nSum until -1:")
while i < len(data) and data[i] != -1:
    total += data[i]
    i += 1

print(f"Sum: {total}")

# Pythonic: itertools.takewhile
from itertools import takewhile
data2 = [10, 20, 30, -1, 40, 50]
total2 = sum(takewhile(lambda x: x != -1, data2))
print(f"Sum (takewhile): {total2}")

Use while when you don't know how many iterations ahead of time.

while Condition-controlled loop: unknown iterations, check before each.

Emulating do-while

Python doesn't have do-while, but you can emulate it.

do_while_emulate.py 
# Python doesn't have do-while, but here are alternatives:

# Pattern 1: while True with break at end
print("=== Pattern 1: while True + break ===")
choice = 0

while True:
    print("=== Menu ===")
    print("1. Play")
    print("2. Settings")
    print("3. Exit")
    print(f"Choice: {choice}")
    
    # Simulate getting input
    choice = 3
    
    if choice == 3:
        break

print("Goodbye!")

# Pattern 2: Loop-and-a-half
print("\n=== Pattern 2: Loop-and-a-half ===")
attempts = 0

while True:
    attempts += 1
    print(f"Attempt {attempts}")
    
    # Simulate: succeed on 3rd try
    success = (attempts == 3)
    
    if success:
        print("Success!")
        break
    
    print("Retrying...")

# Pattern 3: Assignment expression (walrus operator)
print("\n=== Pattern 3: Walrus operator ===")
numbers = iter([1, 2, 3, 0, 4, 5])

while (n := next(numbers, None)) is not None and n != 0:
    print(f"Processing: {n}")

print("Stopped at 0 or end")

Use while True with break at the end, or loop-and-a-half pattern.

enumerate `for i, item in enumerate(list)` - get index and value together.
zip `for a, b in zip(list1, list2)` - iterate multiple lists in parallel.

Exercise: infinite_patterns.py

Explore intentional infinite loops and how to use them safely