Worksheet

9.3 Protocols

Talking to each other · Robot club · about 20 min

What this lesson is about

Agreeing what a message looks like; splitting it into words and numbers.

• Asking and waiting
• Taking a message apart
• Strict out, forgiving in

Questions 7 marks in all

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

   words = "forward 30".split()
   command, amount = words[0], float(words[1])
   print(command, amount * 2)

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

   def numbers_in(text):
       out = []
       for word in text.replace(",", " ").split():
           try:
               out.append(float(word))
           except ValueError:
               pass
       return out
   
   print(numbers_in("ball at -12,40 ok"))

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

   text = "  Forward 30 "
   words = text.lower().strip().split()
   print(words)

4. [1 mark]Why does ask give up after a second instead of waiting for a reply forever?

   1. ARadios miss things, and a program waiting for a reply that never comes is stuck
   2. BThe Guide only speaks for one second
   3. Cmessages() stops working after a second
   4. DTo save battery

5. [1 mark]A message arrives saying jump 10, which your program does not understand. What should it do?

   1. AIgnore it and carry on
   2. BStop with an error so you notice
   3. CTreat it as forward 10
   4. DSend it back to whoever sent it

6. [1 mark]ask("next") returns None. What does that mean?

   1. ANo reply arrived within the time allowed
   2. BThe Guide said none
   3. CThe robot has reached the green zone
   4. DThe message was sent to nobody

7. [1 mark]What is the word for the agreement, made in advance, about what messages look like?

The task: follow instructions

Ask the Guide for instructions one at a time, do each one, and stop at done. You will end up in the green zone.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()
def ask(question, seconds=1.0):
    # send a question and return the first reply that arrives within `seconds`, or None
    send(question)
    for tick in range(int(seconds * 10)):
        # pause 0.1 s (the robot keeps doing what it was told)
        wait(0.1)
        for sender, text in messages():
            return text
    return None

def numbers_in(text):
    # every number in a message, as floats: "at 25,55" -> [25.0, 55.0]
    out = []
    for word in text.replace(",", " ").split():
        try:
            out.append(float(word))
        except ValueError:
            pass
    return out
reply = ask('next')
print('guide says:', reply)

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/9-3-protocols/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Handle left <cm> and backward <cm> too, even though this Guide never sends them.
2. Handle turn <degrees>.
3. If ask returns None, try again, up to three times, then give up and print why.