Answers

9.5 Taking turns

Talking to each other · Robot club · about 20 min

What this lesson is about

One robot at a time through a corridor: announce, wait, go.

• Listen to the rhythm
• Wait, announce, go
• Straight down the middle

Questions 6 marks in all

1. [1 mark]In the corridor task, which message tells you it is time to go?

   1. Acorridor clear
   2. Bcorridor busy
   3. Centering
   4. Dout
   Answer: A. The Porter says busy as it enters and clear as it leaves. Wait for clear.

2. [1 mark]The corridor stays clear for about 9 seconds, and you need about 6 seconds to get through. After hearing clear, how many seconds can you afford to delay before setting off?

   Answer: 3 (accept within 0.5). 9 - 6 = 3 seconds, which is why you go straight away.

3. [1 mark]What does this program print?

   batches = [[("Porter", "corridor busy")], [], [("Porter", "corridor clear")]]
   clear = False
   checks = 0
   for batch in batches:
       checks += 1
       for sender, text in batch:
           if "clear" in text:
               clear = True
       if clear:
           break
   print("clear" if clear else "waiting", "after", checks, "checks")

   Answer:

   clear after 3 checks

   Each batch is one call to messages(). The first has busy, the second nothing, and the third has clear, so it stops after 3 checks.

4. [1 mark]Put the steps for getting through the corridor in order.

   Number the lines 1 to 4 to put them in the right order.

   1. Drive through, holding the middle
   2. Wait for corridor clear
   3. send("entering")
   4. send("out")
   Answer:

   Wait for corridor clear
   send("entering")
   Drive through, holding the middle
   send("out")

   Wait, announce, go, and say when you are through.

5. [1 mark]The Porter does not listen to messages. Why send entering anyway?

   1. AA protocol is the agreement: a robot that did listen could hold back for you
   2. BThe Porter stops if it hears it
   3. CThe checker ignores robots that do not send
   4. Dforward only works after a send
   Answer: A. The agreement is what matters, not the hardware at the other end.

6. [1 mark]Driving along the corridor, the robot faces heading 90. Why does it steer from y and not x?

   1. AFacing along x, sideways drift shows up in y
   2. Bx is always zero
   3. Cposition() only gives y
   4. DHeading 90 means facing up the mat
   Answer: A. Heading is clockwise from up the mat, so 90 faces along x, and y is how far off the centre line it is.

The task: one at a time

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()
# radio this to every other robot
send('entering')
# drive forward at 60 for 72 cm, then stop
forward(60, distance=72)
# radio this to every other robot
send('out')

from bugbot import *
connect()
def wrapped(h):
    return (h + 180) % 360 - 180

clear = False
while not clear:
    for sender, text in messages():
        print(sender, "says:", text)
        if "clear" in text:
            clear = True
    wait(0.1)
send("entering")
while position()[0] < 72:
    x, y = position()
    drive(60, y * 6, wrapped(90 - heading()) * 3)     # straight down the middle of the corridor
    wait(0.1)
stop()
send("out")