Lecture Notes — Lecture 04: First Python: State and Sequences

Companion to the lecture plan and the worked examples. Read the Definitions before class, the Explanation after it; the practice questions and the lecture quiz close the loop.

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.
• Index — the position of an item; scores[0] is the first; the last of n items is at n-1.
• input() — always returns a string; convert before arithmetic.

Explanation

Python as executable pseudocode. The course grammar maps line to line: READ → input(), WRITE → print(), x <- 5 → x = 5, IF/ELSE and WHILE look nearly the same. New this week is only the punctuation.

State, names, and order. Python runs top to bottom; using a name before assignment is an error, not zero. Reassignment x = x + 1 is the pseudocode x <- x + 1 exactly.

Types are safety rails. input() gives strings; "3" * 4 is '3333', not 12. Convert deliberately: int(...), float(...). When something odd happens, the first question is 'what type is this really?'

Lists as sequences. Positions start at 0; len() counts; for item in lst visits every element. The last valid index is len(lst) - 1 - remember this number, lecture 6 will show you every way to get it wrong.

First debugging tool. print() between lines is the honest first debugger: show the state you assumed, compare with reality. (The editor's debugger arrives in the lab.)

Examples

The lecture's worked examples live in examples.md; the case studies this lecture builds on are worked in class from the projector. The case sequence with difficulties:

• cs-013 (Beginner)
• cs-014 (Foundational)
• cs-015 (Intermediate)
• cs-016 (Advanced)

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 and key takeaways

1. Predict, then run, then check - never just run.
2. input() gives strings; convert before arithmetic.
3. Lists start at index 0; the last index is len-1.
4. print() the state you assumed; compare with the state you got.

Practice questions

• Write a program that reads three numbers and prints their average, rounded to one decimal.
• Read numbers until 0 and print the count of negatives and the sum of positives.
• Given scores = [8, 6, 9, 7], predict print(scores[1], scores[-1], len(scores)) - then run it.
• Break the echo program with the nastiest legal input you can construct; explain the failure precisely.

Where this leads

Next lecture: Numbers, Remainders, and Cycles. The quiz below checks this lecture's essentials before we build on them.