A student council treasurer has a small, real problem. The council saves a fixed amount each week toward one purchase, and she is asked, constantly, “how much will we have by week such-and-such?” She would like a printed schedule she can pin to the wall.
Here is the program she has. It handles two weeks.
week = 1
balance = 25.00
print(f"Week {week}: ${balance:.2f}")
week = 2
balance = balance + 7.50
print(f"Week {week}: ${balance:.2f}")Run it. It prints exactly what you would expect:
Week 1: $25.00
Week 2: $32.50
The task
She needs one hundred weeks.
That is the whole task. You know everything you need — assignment,
addition, print, f-strings — and there is nothing clever available to
you yet. Get her the hundred weeks. Copy, paste, edit. Start the clock.
Nobody is being tricked here. You are being timed, and the number that matters is not how fast you finish but where you are at the fifteen-minute mark.
The change request
At some point during the work — your teacher chooses the moment, and it will not be a convenient one — the treasurer sends an update:
Sorry! The weekly amount changed. It is 7.50. And we start from 25.
Do not groan quietly. Groan out loud, and then answer three questions in writing, because the answers are the lesson:
- How many lines do you now have to change?
- How many of them will you miss?
- If she changes it again next week, what exactly is your plan?
Facilitation notes
Do not mention loops. Not once, not as a hint, not as a wink. The word arrives at the end, from the room, as a relief. If a student already knows the word, give them the extension below and swear them to secrecy — they will enjoy the conspiracy.
Timing in a 70-minute period. Five minutes to set the scene and run the two-week program; fifteen of grim copy-paste; the change request at minute twenty, sprung with theatrical apology; ten minutes of repair and the three questions; five for the count below; then twenty-five for the consolidation into Repetition and a first loop written together.
The count. Before naming anything, poll the room: how many lines did you write? How many of you got all hundred correct? How many found a mistake only after the change request? The honest numbers on the board are more persuasive than any explanation of why loops exist.
What to listen for. “There has to be a way to just tell it to do that a hundred times.” That sentence, from a student, is the lesson arriving on schedule. Write it on the board with their name beside it, and build the first
forloop from their words.Extension for the fast or the forewarned. Ask for the last week the balance is under $500, without printing the schedule at all. That quietly needs a condition inside the repetition, which sets up the accumulator work later in the unit.
What tends to surface
The pain is not the typing. The pain is the change request, and the discovery that a hundred copies of an idea means a hundred chances to be wrong about it. Somebody will notice that only two things differ between one block and the next — the week number and the balance — and that everything else is identical. That observation is the concept. The name is the easy part.
You also meet a professional truth early: the requirements changed halfway through, which is not a cruel trick but a description of the job. It happens to your community client too, and The Software Development Process has a whole vocabulary for it.
Where this goes next
Everything you did by hand this morning collapses into about three lines. The name for those three lines is in Repetition, a working example is in Looping Programs, and the mechanics get drilled in Loops Practice. The treasurer, for the record, gets her schedule — and by the end of Unit 2 she gets something she can act on, which is what The Data Digest is about.
The answer is not on this page
The three-line version is not printed here, deliberately. Your class writes it together, from the sentence somebody in the room says out loud when the copying becomes unbearable. If you skip ahead and paste a loop from the internet today, you will have the code and not the reason — and the reason is the only part that is hard to get back.
Curriculum connection
A2.2
use sequence, selection, and repetition control structures to create programming solutions;
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B1.1
use various problem-solving strategies (e.g., stepwise refinement, divide and conquer, working backwards, examples, extreme cases, tables and charts, trial and error) when solving different types of problems;
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