Lesson 35

Card 4.5: State Machines

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Format: Design discussion + paper artifact

Time: 40 min

Prerequisites: Ladder rung 3; Card 3.5 pairs well

Core Question

The float’s mission is a sequence: wait, descend, hold, ascend, surface, transmit, repeat. Code that handles this as one long tangle of ifs becomes unfixable. What is the disciplined shape?

Resource (~10 min)

A state machine in plain terms: the system is always in exactly one named state; events or conditions cause transitions; each state has its own simple job. Whiteboard example: a traffic light, then a microwave (door open beats everything, which introduces guard conditions).
Look at the float mission description in the manual and list every distinct phase you can find.

Activity

Human state machine warm-up (5 min): three volunteers are states, the class calls out events, volunteers point to who becomes active. Fast, silly, and it works.
Real artifact: draw the complete state diagram for the float’s mission on paper: named states in circles, labeled transitions with their triggering conditions (“depth within 0.2 m of target for 5 s” beats “when it gets there”), including at least one failure path (what state do you enter if the target depth is never reached?).

Clearing This Card

The state diagram, plus a 90-second oral check: the teacher names a weird moment (“the float is ascending and the pressure sensor starts returning garbage”) and you trace what your machine does and whether that is acceptable.

If You Miss This Class

Mission description and a traffic-light example diagram are on the class page; same artifact and check.

Why This Matters

The float code’s structure is this diagram, and the diagram belongs in the technical documentation, where a clean state machine figure impresses judges far beyond its effort. Software team members will implement it; every team member should be able to read it.