Answers

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)

   Answer:

   forward 60.0

   split() gives the words forward and 30. float turns the text 30 into the number 30.0, which can do sums.

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"))

   Answer:

   [-12.0, 40.0]

   Commas become spaces, so the words are ball, at, -12, 40 and ok. Only -12 and 40 turn into numbers.

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

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

   Answer:

   ['forward', '30']

   lower() makes Forward into forward and strip() removes the spaces at the ends. Forgiving in.

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
   Answer: A. The timeout is part of the protocol. None means no answer came in time.

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
   Answer: A. Be strict about what you send and forgiving about what you receive. A robot that crashes on an odd message is not much of a team mate.

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
   Answer: A. None is not an instruction. Check for it before calling split(), or the program crashes.

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

   Answer: protocol. Here the protocol is one word then a number, like forward 30, with done at the end.

The task: follow instructions

# 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)

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)):
        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
while True:
    reply = ask("next")
    print("guide says:", reply)
    if reply is None:
        continue
    words = reply.split()
    if words[0] == "forward":
        forward(50, distance=float(words[1]))
    elif words[0] == "right":
        right(50, distance=float(words[1]))
    elif words[0] == "left":
        left(50, distance=float(words[1]))
    elif words[0] == "done":
        break
stop()