Lunch for 30 people

You must make lunch for 30 people by 12 o'clock. It feels too big.

So you split it: make a shopping list, buy food, cook, serve, clean up. Each job is small. Each job has one owner.

Developers do the same with a big brief.

Key words

Word	What it means	Example
Decomposition	Splitting a big problem into small parts.	booking tool: book, cancel, price
Subtask	A small job inside a bigger job.	check the hour
User story	One need, told from the user's side.	As desk staff, I want to see the day's bookings
Function	A named piece of code that does one job.	cancel()
Abstraction	Leaving out details you do not need.	a booking is a name, court and hour
Refinement	Adding more detail to a part, one level at a time.	check the hour: 9 to 20 only
Dependency	A job that must finish before another starts.	design before build

The cells students fill in are left blank in this preview.

How small is small enough?

A good piece is small enough to do in one sitting. It does one thing. You can test it alone.

Ask: "Can I say what this piece does in one short sentence?" If you cannot, split it again.

Refine: add detail in rounds

Do not write every detail at once. Work in rounds. This is refinement.

Round 1: "Make a booking." Round 2: "Check the hour, check the court, save." Round 3: "The hour must be from 9 to 20."

Each round adds detail and hides nothing. Write one line on why you split the problem that way. A marker rewards a reason for the split, not only the split.

Try it: each part is a function

Code: python

def check_hour(hour):
    return hour >= 9 and hour <= 20


def price_for(is_member, hours):
    if is_member:
        rate = 4
    else:
        rate = 6
    return rate * hours


print(check_hour(10))
print(check_hour(21))
print(price_for(True, 2))

# Call price_for for a visitor for 3 hours.

User stories: needs from the user's side

A user story is one sentence. It has three parts: who, what, why.

"As a desk worker, I want to see all of Saturday's bookings, so that I can tell a caller which hours are free."

One story is one need. Stories are easy to test.