Lecture II - Control Structures for Your Code

Programming with Python

Author
Affiliation

Dr. Tobias Vlćek

Kühne Logistics University Hamburg - Fall 2026

Episode 2: The Curfew

A decree from the city

Overnight the city banned delivery after 22:00. Your app can no longer just say yes to every order. Today the code starts making decisions, repeating work over every order, and tidying up Tobi’s menu.

. . .

(Tobi’s fix: “we just deliver yesterday’s orders the next morning.” A lawyer disagreed.)

🔥 Warm-up

Three questions from Episode 1. Commit. Hands up before the reveal.

Question 1

price = 8.90
print(f"{price * 2:.2f}")

a) 17.8 b) 17.80 c) Error

Answer 1

b): :.2f always prints two decimals. That’s the whole receipt trick.

Question 2

print(type("9.99"))

a) float b) int c) str

Answer 2

c): quotes make it text, no matter how numeric it looks. (Tobi learned this the hard way.)

Question 3

print(7 // 2, 7 % 2)

a) 3.5 1 b) 3 1 c) 3 0.5

Answer 3

b): // floors, % gives the remainder. Together: “how many fit, what’s left.”

Making Decisions

Asking a yes/no question

A comparison asks a question and answers with a boolean: True or False, the bool type you met last week:

delivery_hour = 21
print(delivery_hour < 22)    # before curfew?
print(delivery_hour == 22)   # exactly at curfew?
True
False

The comparison operators

Six operators, all returning True or False:

  • < less than · > greater than
  • <= at most · >= at least
  • == equal · != not equal

. . .

= assigns a value; == compares two values. Mixing them up is the classic first-week bug.

Combining conditions

and, or, not glue booleans together:

is_weekday = True
before_curfew = 21 < 22
print(is_weekday and before_curfew)   # both must be True
True

. . .

and needs both sides true; or needs at least one; not flips it.

if: do something only when

An if runs its indented block only when the condition is True:

delivery_hour = 21
if delivery_hour < 22:
    print("Open: send the courier.")
Open: send the courier.

. . .

The indentation (4 spaces) is what marks the block. Python is strict about it.

if / else: two paths

else catches every case the if missed:

delivery_hour = 23
if delivery_hour < 22:
    print("Open: send the courier.")
else:
    print("Curfew: kitchen closed.")
Curfew: kitchen closed.

if / elif / else: a ladder

More than two cases? Stack elif branches. Reward loyal customers by tier:

past_orders = 8
if past_orders >= 20:
    tier = "Gold"
elif past_orders >= 5:
    tier = "Silver"
else:
    tier = "Bronze"
print(tier)
Silver

Predict: Tobi reorders the ladder

Tobi rewrote the tier ladder “to keep the common case on top”. A customer with 25 past orders walks in. Which tier do they get?

past_orders = 25
if past_orders >= 5:
    tier = "Silver"
elif past_orders >= 20:
    tier = "Gold"
else:
    tier = "Bronze"
print(tier)

a) Gold b) Bronze c) Silver

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: the first true branch wins

c) Silver: Python checks branches top to bottom and takes the first True one. 25 >= 5 is already True, so the Gold branch is never even looked at. Order your ladder from strictest to loosest.

past_orders = 25
if past_orders >= 5:
    tier = "Silver"
elif past_orders >= 20:
    tier = "Gold"
else:
    tier = "Bronze"
print(tier)
Silver

. . .

(One misplaced elif and Tobi demotes every VIP.)

Predict: exactly at curfew

The curfew is 22:00. An order comes in at exactly 22:00. What prints?

delivery_hour = 22
print(delivery_hour < 22)

a) False b) True c) Error

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: 22 < 22

a) False: < is strict. 22 is not less than 22, so at 22:00 sharp the kitchen is already closed. Use <= when the boundary should count.

delivery_hour = 22
print(delivery_hour < 22)
False

Your turn — 10 minutes

Open the exercise (scan the QR or type the link):

python.tobiasvlcek.com/notebooks/ex_02_a/

First predict what happens, then run it.

Doing Things Many Times

The copy-paste pain

Tobi totals the day’s orders by hand, one print per order:

print("Falafel Wrap")
print("Pad Thai")
print("Miso Ramen")
Falafel Wrap
Pad Thai
Miso Ramen

. . .

Three lines for three items. A hundred orders? A hundred lines. There is a better way.

for: one step for every item

A for loop runs its block once for each item in a list:

menu = ["Falafel Wrap", "Pad Thai", "Miso Ramen"]
for item in menu:
    print(item)
Falafel Wrap
Pad Thai
Miso Ramen

. . .

item is the loop variable. It takes each value in turn, and the name is yours to choose.

The accumulator pattern

Start a total at 0, then add each item as the loop visits it:

prices = [5.00, 3.50, 6.50]
total = 0
for price in prices:
    total = total + price
print(total)
15.0

. . .

After the loop, total holds the sum. This is how you replace Tobi’s hand-counted dashboard.

range(): count without a list

range(n) counts from 0 up to, but not including, n:

for i in range(3):
    print(i)      # ping the courier's phone 3 times
0
1
2

. . .

Three passes: 0, 1, 2. (It stops before 3, a beginner’s favorite surprise.)

range() with two and three arguments

  • range(start, stop): begin at start, stop before stop
  • range(start, stop, step): jump by step each time
print(list(range(2, 5)))       # list() shows the range as a list: 2, 3, 4
print(list(range(0, 10, 3)))   # every 3rd order gets a flyer: 0, 3, 6, 9
[2, 3, 4]
[0, 3, 6, 9]

Looping over a string

A string is a sequence too, and a for loop walks it character by character:

for letter in "Pad":
    print(letter)
P
a
d

A loop in one line (a preview)

A list comprehension is a for loop compressed into one line that builds a list:

doubled = [n * 2 for n in [4, 5, 6]]
print(doubled)
[8, 10, 12]

. . .

Just a taste. Session IV gives comprehensions their proper treatment. For now: recognize the shape.

Predict: where does print run?

The print is not indented. What does this show?

total = 0
for n in [10, 20]:
    total = total + n
print(total)

a) 10 b) 10 then 30 c) 30

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: the unindented print

c) 30: print sits outside the loop, so it runs once, after the loop finishes, showing the final total. Indent it and it would print on every pass. Indentation decides what repeats.

total = 0
for n in [10, 20]:
    total = total + n
print(total)
30

Your turn — 10 minutes

Open the exercise (scan the QR or type the link):

python.tobiasvlcek.com/notebooks/ex_02_b/

First predict what happens, then run it.

Repeat Until, and Clean Text

while: repeat as long as

A while loop repeats as long as its condition stays True. Suppose MunchCorp undercuts you. Cut your price each round until you drop below their price:

price = 10.00
rounds = 0
while price >= 7.00:
    price = round(price * 0.8, 2)   # 20% off each round
    rounds = rounds + 1
print(rounds, price)
2 6.4

. . .

Two rounds and you’re under 7.00. (The lab’s price war against MunchCorp is your boss fight after the break.)

The loop must move toward its goal

Every pass must nudge the condition closer to False: here the price shrinks, so the loop is guaranteed to end.

. . .

If nothing changes, the condition never flips and the loop runs forever:

# ⚠️ NEVER run this. It never stops, and freezes the browser tab
count = 1
while count > 0:
    count = count + 1    # count only grows, so the condition stays True forever

. . .

An endless while will freeze your marimo tab. Always check that something inside the loop changes the condition.

break: an early exit

break jumps out of a loop immediately, handy with while True:

count = 0
while True:
    count = count + 1
    if count == 3:
        break
print(count)
3

. . .

The loop would run forever, but break stops it the moment count hits 3.

Cleaning up text

Tobi typed the daily special IN ALL CAPS with stray spaces. Strings have methods that return a cleaned-up copy:

raw = "  miso ramen  "
print(raw.strip())          # drop outer spaces
print(raw.strip().title())  # then Capitalise Each Word
print("special".upper())    # SHOUT
print("MOIN".lower())       # whisper
miso ramen
Miso Ramen
SPECIAL
moin

. . .

Methods can be chained left to right: raw.strip().title().

Trimming specific characters

.strip() drops spaces; .rstrip("!") drops trailing !. Chain them to fix Tobi’s shouting:

print("SALE!!!".rstrip("!"))            # SALE
print("SALE!!!".rstrip("!").title())    # Sale
SALE
Sale

. . .

The original string is never changed. Each method hands back a new one.

Predict: chained methods

What does this print?

print("  MOIN  ".strip().lower())

a) MOIN b) moin c) moin

. . .

Predict first. Pick a letter, then I reveal the answer.

Answer: " MOIN ".strip().lower()

b) moin: .strip() removes the outer spaces, then .lower() lowercases the result. Chained methods run left to right, each acting on the previous one’s output.

print("  MOIN  ".strip().lower())
moin

Your turn — 10 minutes

Open the exercise (scan the QR or type the link):

python.tobiasvlcek.com/notebooks/ex_02_c/

First predict what happens, then run it.

📋 Checkpoint 0: dress rehearsal

A practice board review. Fifteen minutes, before the lab, ungraded. Same format as the five real checkpoints, so that nothing is new next week when it counts.

  • Individual work, as in the real thing: no AI, no neighbors, no chat
  • The link and QR are handed out in class. Open it and start
  • 3 short tasks: compute a price, trace a line, fix one of Tobi’s bugs
  • Fill in your name and student ID as code at the top, exactly as you will next week

When you’re done

Menu → Download → Download Python code → upload the .py to the “Checkpoint 0” assignment on Moodle. Nothing is graded today: the point is that download and upload are muscle memory by Session III.

. . .

Next week this exact routine is Checkpoint 1: 40 minutes, 12 points, Sessions I–II. Today you get to make every mistake for free.

To the Lab

After the break: the lab

  • Head to the lab notebook: Episode 2 — The Curfew
  • You’ll enforce the 22:00 curfew, total the day’s orders in a loop, de-shout Tobi’s menu, and fight the MunchCorp price war with a while loop
  • It runs entirely in your browser: no setup, just click and code

. . .

Download your .py before you leave. Closing the tab without downloading loses your work, and downloading is exactly how you hand in the checkpoints.

Wrap-up

Three things to remember

  1. if / elif / else run the first true branch: test the strictest condition first
  2. for repeats over every item (accumulator: start at 0, add each); while repeats until its condition flips, and something inside must move it there
  3. String methods (.strip(), .title(), .upper(), .rstrip("!")) return a cleaned copy, and chain left to right

. . .

Next time — Episode 3 starts with Checkpoint 1: 40 minutes, no AI, everything from Episodes 1–2, so keep this notebook and the last one close. Then: Tobi has pasted the same receipt code 14 times, and one small change now takes him an afternoon. We bring in functions to the rescue.

Literature

Books to start with

  • Downey, A. B. (2024). Think Python: How to think like a computer scientist (Third edition). O’Reilly. Link to free online version
  • Elter, S. (2021). Schrödinger programmiert Python: Das etwas andere Fachbuch (1. Auflage). Rheinwerk Verlag.

. . .

Nothing new here, but these are still great books to start with!

. . .

For more, see the literature list of this course.