Answers

6.7 Where will it be

Seeing more · Robot club · about 20 min

What this lesson is about

Two sightings give a velocity: give way to a robot crossing your path.

• A robot that crosses your path
• Give way
• Predicting

Questions 7 marks in all

1. [1 mark]Robot 102's bearing sweeps from -36 to 33 degrees while its distance only goes from 58 cm down to 40 cm and back out. What is it doing?

   1. ACrossing in front of you
   2. BDriving straight at you
   3. CDriving away from you
   4. DStanding still
   Answer: A. A bearing sweeping through zero while the distance stays about the same is the pattern of a robot crossing your path.

2. [1 mark]What does this program print?

   last = None
   for bearing in [-30.0, -20.0, -8.0, 5.0, 18.0]:
       if last is not None:
           print(bearing, "coming" if abs(bearing) < abs(last) else "leaving")
       last = bearing

   Answer:

   -20.0 coming
   -8.0 coming
   5.0 coming
   18.0 leaving

   Comparing sizes with abs ignores which side it is on. The bearing shrinks until it crosses the middle, then grows.

3. [1 mark]The first give way rule stops whenever robot 102 is close and in front. What is wrong with it?

   1. AIt also stops for a robot that is already moving away from its path
   2. BIt never stops at all
   3. CIt drives into the robot
   4. DIt only works on the left side
   Answer: A. It is safe but too cautious. Only a robot heading for your path is a problem.

4. [1 mark]The rule is tags and tags[0][3] < 30 and abs(bearing_of(tags[0][1])) < 35. Robot 102 is 25 cm away at a bearing of -40. Does the robot stop?

   1. ANo: 40 degrees is outside the 35 degree window
   2. BYes: it is closer than 30 cm
   3. CYes: the bearing is negative
   4. DNo: abs makes the bearing 0
   Answer: A. Both tests must be true. It is close enough, but too far off to the side to count as in the way.

5. [1 mark]When robot 102 is out of view, the prediction code sets last = None. Why?

   1. AAn old bearing from before it was lost would give a wrong comparison next time
   2. BNone makes the robot stop
   3. COtherwise the tag detector switches off
   4. DTo count how long it was lost
   Answer: A. A comparison only means something between two sightings in a row. Starting fresh avoids a false alarm.

6. [1 mark]waited goes up by 1 on every tick the robot stops, at ten ticks a second. At the end waited is 23. What does waited / 10 print?

   Answer: 2.3 (accept within 0.05). 23 ticks at a tenth of a second each is 2.3 seconds of waiting.

7. [1 mark]What does the general version of prediction, used by cars and ships, do?

   Tick every answer that is true.

   1. AKeep the last few positions of the other robot
   2. BWork out its velocity from them
   3. CCheck whether the two paths cross in the next second or two
   4. DStop for every robot it can see
   Answer: A, B, C. Position history gives velocity, and velocity tells you where it is about to be. Stopping for everything is what prediction avoids.

The task: give way

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

# drive forward at 50 for 80 cm, then stop
forward(50, distance=80)

from bugbot import *
connect()
for tick in range(390):
    x, y = position()
    if y > 80:
        break
    seen = [r for r in robots() if r[0] == 'red']
    in_the_way = False
    if seen:
        cx, dist = seen[0][1], seen[0][3]
        bearing = (cx - 160) * 120 / 320
        if dist < 40 and abs(bearing) < 40:      # close and in front of me
            in_the_way = True
    if in_the_way:
        stop()                                   # give way: let it pass
    else:
        forward(50)
    wait(0.1)
stop()