Answers

10.3 Watching the others

Competing · Robot club · about 14 min

What this lesson is about

others() and the camera tags, and giving way early so contact never costs you the game.

• Where everyone is
• Giving way

Questions 6 marks in all

1. [1 mark]In the traffic task, a student calls others() to find the moving robots. What happens?

   1. AIt does not work: others() is only there in games, so in a task use the camera
   2. BIt lists both robots with their positions
   3. CIt returns what the camera can see
   4. DIt sends a message asking where they are
   Answer: A. others() is the game's own view. In a task, robots() shows what is in front of you, each one found by its dome colour.

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

   import math
   bots = [{"name": "red-1", "x": 30, "y": 40}, {"name": "blue-2", "x": -6, "y": 8}, {"name": "blue-3", "x": 20, "y": 0}]
   closest, best = None, 1e9
   for bot in bots:
       d = math.hypot(bot["x"], bot["y"])
       if d < best:
           closest, best = bot["name"], d
   print(closest, best)

   Answer:

   blue-2 10.0

   Measured from (0, 0) the distances are 50, 10 and 20, so blue-2 is closest at 10.0 cm.

3. [1 mark]You touch another robot that drove into you. Whose fault is it, in most games?

   1. AYours: touching another robot is your fault, whoever moved
   2. BTheirs, because they moved
   3. CNobody's
   4. DWhoever has the lower score
   Answer: A. That is why seeing other robots early and giving way matters.

4. [1 mark]Which habits keep you out of trouble on a busy mat?

   Tick every answer that is true.

   1. ALook before you commit
   2. BGo round with left() or right()
   3. CGive way early
   4. DSpeed up to get past before they arrive
   5. EPush through, since they moved into you
   Answer: A, B, C. A quick look, a sidestep and a little slowing cost almost nothing. A collision costs the game.

5. [1 mark]robots() comes back as [['red', 160, 118, 35.0]]. What is 35.0?

   1. AHow far away that robot is, in centimetres
   2. BIts bearing in degrees
   3. CHow big its dome looks, in pixels
   4. DIts id
   Answer: A. Each robot is [colour, cx, cy, distance], nearest first. The bearing comes from cx: (cx - 160) * 120 / 320.

6. [1 mark]What does this program print?

   for gap in [40, 25, 12]:
       if gap < 25:
           print(gap, "step aside")
       else:
           print(gap, "carry on")

   Answer:

   40 carry on
   25 carry on
   12 step aside

   gap < 25 is false when the gap is exactly 25, so only 12 is close enough to step aside.

The task: through the traffic

from bugbot import *
connect()

while position()[1] < 80:
    forward(50)
    wait(0.1)

stop()

from bugbot import *
connect()
def robot_near():
    return any(r[3] < 30 for r in robots())
while position()[1] < 76:
    if robot_near():
        stop()
    else:
        forward(45)
    wait(0.1)
stop()