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Module 5 · Dijkstra's Algorithm

Lesson 7 · Obstacle Detection with the Rangefinder — Knowledge ChecksAnswer key

Correct answers are marked and the explanation follows each question.

  1. How does an ultrasonic rangefinder measure distance?

    1. A.It uses a camera to estimate size
    2. B.It sends a sound pulse and times how long the echo takes to return✓
    3. C.It counts wheel rotations
    4. D.It reads a GPS signal

    Sound pulse out, echo back, time it — that round-trip time converts to a distance. The XRP returns it in centimeters.

  2. Why use a threshold instead of checking for an exact distance?

    1. A.Exact distances are illegal
    2. B.Sensor readings vary slightly (noise), so a cutoff is more reliable than an exact match✓
    3. C.To make the robot slower
    4. D.Thresholds use less memory

    Ultrasonic readings fluctuate with angle, surface, and noise. A threshold ('closer than 15 cm = blocked') absorbs that variation; an exact-match check would fail constantly.

  3. The robot is at (2,1) facing NORTH and the rangefinder reads 8 cm (threshold 15). Which node is blocked?

    1. A.(2,1) — where the robot is
    2. B.(1,1) — the node directly ahead (north is row − 1)✓
    3. C.(3,1)
    4. D.(2,2)

    Facing north means the next node is row − 1: (1,1). The robot sits on a clear node; the obstacle is the adjacent one it faces.

  4. When the robot detects a new obstacle, what does it do to reroute?

    1. A.Stops permanently
    2. B.Creates a fresh Dijkstra with the updated blocked list and current position, then recomputes the path✓
    3. C.Reverses back to the start
    4. D.Ignores it and drives through

    Detecting an obstacle means reroute, not stop. A new Dijkstra with the updated blocked list and the robot's current position produces a path around it.

  5. Why test with a simulated rangefinder before using the real one?

    1. A.The real sensor never works
    2. B.To verify the detect-update-recompute logic in software first, separating code bugs from hardware bugs✓
    3. C.Simulation is required by Python
    4. D.To avoid writing the loop

    A simulated sensor lets you confirm the rerouting logic on screen. Once that's solid, real-sensor problems are clearly hardware, not logic — much easier to debug.