Lecture Notes — Lecture 03: Flowcharts and Control Flow

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

• Flowchart — a diagram of control flow: processes in rectangles, decisions in diamonds, one start and one end.
• Process box — a rectangle: one action or computation.
• Decision diamond — exactly two labelled exits (true/false); both must be reachable.
• Terminator — the rounded start/end nodes; every path eventually reaches the end.
• Connector — a labelled circle joining arrows across the page (avoids spaghetti).
• Dead branch — a path that no input can ever take - usually a bug in the conditions.
• Termination argument — the reason a loop must stop: some quantity shrinks every pass and has a floor.

Explanation

What flowcharts add over pseudocode. Termination and reachability become visible: a diamond with one exit, an arrow into a dead end, a loop whose exit condition can never be true - all obvious in a picture, easy to miss in text.

The arrow discipline. Every diamond has exactly two labelled exits. Every path from start reaches end. Nothing floats unattached. Four symbols total (terminator, process, decision, connector) - resist flowchart libraries with 20 shapes.

From words to diamonds. Each 'otherwise', 'unless', and 'until' in the requirements is a decision. Extract them first, then wire the normal path around them.

Tracing arrows = tracing state. Walk a real input through the arrows with a finger. If you cannot say which branch a concrete input takes, the flowchart (or the spec) is broken.

State machines. A vending machine is four states (idle, accepting credit, vending, sold-out) and the arrows are the events. This is the flowchart idea wearing a different hat - and it returns in cs-007 and lecture 12's clinics.

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-009 (Beginner)
• cs-010 (Foundational)
• cs-011 (Intermediate)
• cs-012 (Advanced)

Common misconceptions

• Diamonds with one exit or unlabelled arrows - half the class does this on day one.
• A loop whose exit arrow leaves from a process box - exit decisions belong in diamonds.
• Trusting the picture without tracing a concrete input through it.
• Drawing 20 symbols where 5 suffice - diagrams are for clarity, not decoration.

Summary and key takeaways

1. Four symbols, two exits per diamond, every path reaches the end.
2. Every loop names the quantity that shrinks.
3. Trace arrows with a real input before believing the picture.
4. Flowcharts expose dead branches and bad termination that text hides.

Practice questions

• Draw the flowchart for 'keep reading numbers until 0, then print the count of negatives'.
• Mark the two exits on every diamond of your ATM flowchart; name what shrinks for the loop.
• Find the dead branch in a provided discount flowchart (10% over 100, 20% over 200, else 5%).
• Trace the guess-the-number flowchart with target 7 and guesses 4, 8, 7 - list every arrow taken.

Where this leads

Next lecture: First Python: State and Sequences. The quiz below checks this lecture's essentials before we build on them.