Lecture 04 — First Python: State and Sequences

Learning objectives

• Translate pseudocode into runnable Python
• Explain variables as named state
• Use input/output and simple sequence iteration

Case-based lecture: you attempt, we discuss, then we reveal.

The plan

• Four cases: cs-013, cs-014, cs-015, cs-016
• Each: 5-minute attempt → discussion → reveal → prompt
• Worked examples in between — ask anything
• Break mid-lecture · exit question at the end

Key definitions

• Variable — a name bound to a value; reassignment re-binds the name, it does not change history.
• Expression — a piece of Python that produces a value (`2 + 3 * 4`, `len(name)`).
• Statement — an instruction (`x = 5`, `print(x)`) - note the difference from an expression.
• Type — the kind of value: int, float, str (text), bool. `"3" + 4` fails because types disagree.
• Type conversion — `int(...)`, `float(...)`, `str(...)` translate between kinds; `int("3.5")` fails.
• List — an ordered, changeable sequence: `scores = [8, 6, 9]`; positions start at 0.

Echo with limits, translated from pseudocode

lines = ["coffee", "x" * 21, "tea", "done", "late"]
skipped = 0
for line in lines:
    if line == "done":
        break
    if len(line) > 20:
        skipped += 1
    else:
        print(line.upper())
print(skipped)

Expected: COFFEE then 1

Cost: O(total input length). Python: `while True:` + `break` mirrors the flowchart's loop with two exits.

Course list cleaner: first lists, first filters

Expected: ['CS1', 'MA2'] 2

Cost: O(n^2) with the list-membership check; the set version (lecture 13) is O(n) - say so, then wait.

The change maker: state and arithmetic

Expected: credit 250, change 15 -> [10, 5]

Cost: O(coins + denominations). The greedy breakdown works for this coin system - lecture 23 returns to when greedy is trustworthy.

The meeting scheduler: constraints first

Expected: schedule slots 1-4 used; refused: [Friday request]

Cost: O(requests x slots). Greedy first-fit is fine here; question for fast finishers: when does first-fit waste capacity?

• **Input:** input numbers: 4, 9, 2, 9, 0 (sentinel)
• **Output:** count 4, sum 24, largest 9
• Solve for the worked instance first. Then ask what would change if one
• number, name, or condition were replaced by something stranger.

Pseudocode, flowchart or Python. State one assumption. Compare with your neighbour at minute 4.

cs-013 — discuss

• What did the deciding quantity turn out to be?
• Which assumption shaped your answer?
• After the solution is revealed, the class spends five minutes on this prompt: **"Which accumulator was hardest to get right, and what test would have caught the mistake?"**

Instructor: the reveal, the reasoning section and common wrong turns are in the instructor distribution for cs-013 — not embedded in this deck.

• **Input:** [" CS1 ", "DS1", "CS1", " Stats "]
• **Output:** 1. CS1 2. DS1 3. Stats (trimmed, deduplicated, numbered)
• Solve for the worked instance first. Then ask what would change if one
• number, name, or condition were replaced by something stranger.

Pseudocode, flowchart or Python. State one assumption. Compare with your neighbour at minute 4.

cs-014 — discuss

• What did the deciding quantity turn out to be?
• Which assumption shaped your answer?
• After the solution is revealed, the class spends five minutes on this prompt: **"Where exactly would this solution break first if the input grew to a million items — and would anything break at all?"**

Instructor: the reveal, the reasoning section and common wrong turns are in the instructor distribution for cs-014 — not embedded in this deck.

• **Input:** amount 87 cents
• **Output:** 25x3, 10x1, 5x0, 1x2
• Solve for the worked instance first. Then ask what would change if one
• number, name, or condition were replaced by something stranger.

Pseudocode, flowchart or Python. State one assumption. Compare with your neighbour at minute 4.

cs-015 — discuss

• What did the deciding quantity turn out to be?
• Which assumption shaped your answer?
• After the solution is revealed, the class spends five minutes on this prompt: **"Which accumulator was hardest to get right, and what test would have caught the mistake?"**

Instructor: the reveal, the reasoning section and common wrong turns are in the instructor distribution for cs-015 — not embedded in this deck.

• **Input:** busy [(9,10), (11,12.5)]; day 8..18; meeting 1.5 h
• **Output:** starts at 12.5, 13.0, 13.5, ..., 16.5 (9 starts; the 8-9 and 10-11 gaps are each too short for 1.5 h)
• Solve for the worked instance first. Then ask what would change if one
• number, name, or condition were replaced by something stranger.

Pseudocode, flowchart or Python. State one assumption. Compare with your neighbour at minute 4.

cs-016 — discuss

• What did the deciding quantity turn out to be?
• Which assumption shaped your answer?
• After the solution is revealed, the class spends five minutes on this prompt: **"Where exactly would this solution break first if the input grew to a million items — and would anything break at all?"**

Instructor: the reveal, the reasoning section and common wrong turns are in the instructor distribution for cs-016 — not embedded in this deck.

Common misconceptions

• Adding a number to a string ('3' + 4) - types must agree, so convert first.
• Forgetting input() returns strings and 'multiplying' text by accident.
• Off-by-one on the last element (index n of an n-length list).
• Using a variable before any assignment and expecting zero.

Summary & exit question

• Named technique today: First Python: State and Sequences
• Four cases attempted — methods before answers

Exit question (choose one, 60 seconds)

1. What does predict-then-run change about learning?
2. Name the three Python errors you met today and their causes.
3. Why convert input strings before arithmetic?