Listening

messages(): what arrived since you last asked, and who sent it.

9.2Talking to each otherRobot club15 min

Do this lesson in the simulator

Print it: worksheet (PDF) · answers (PDF) · as a web page

messages() returns everything that arrived since you last asked, oldest first. Each message is a pair: who sent it, and what they said.

They queue up

The Beacon on this mat sends tick <n> once a second. Wait a while before asking, and they are all there waiting for you:

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

# pause 3.2 s (the robot keeps doing what it was told)
wait(3.2)
for sender, text in messages():
    print(sender, "said:", text)

Run this in the simulator

for sender, text in messages() unpacks each pair into two names as it goes. Four ticks, sent at 0.1, 1.1, 2.1 and 3.1 s and delivered together, because nobody asked for them sooner. The radio does not lose what you do not read straight away; it keeps it until you do.

Ask often

To act on a message when it arrives, ask in a loop, with a short wait:

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

# do this 50 times (tick counts from 0)
for tick in range(50):
    for sender, text in messages():
        print(f"{clock():.1f} s: {sender} said {text}")
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)

Run this in the simulator

Now each tick is printed within a tenth of a second of being sent. This shape, check, act, wait, is called polling, and it is how every robot in this module listens. The wait matters: without it the loop asks thousands of times a second and the robot has no time for anything else.

Whose message?

Everyone hears everything, so on a busy mat the sender matters. Ignore what is not for you:

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

# do this 30 times (tick counts from 0)
for tick in range(30):
    for sender, text in messages():
        if sender == "Beacon":
            print("beacon:", text)
        else:
            print("ignoring", sender)
    # pause 0.1 s (the robot keeps doing what it was told)
    wait(0.1)

Run this in the simulator

Task: listen

Print heard <sender>: <text> for each of the Beacon's messages, at least five of them. Do not drive.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()
# pause 2 s (the robot keeps doing what it was told)
wait(2)
print(messages())

Challenges

  1. Count the ticks and print the total at the end.
  2. Print the time between one tick and the next, using clock().
  3. Stop listening as soon as tick 5 arrives.