Answers

5.3 Timers

Behaviours · Robot club · about 15 min

What this lesson is about

Two things at once, remembered ticks, timeouts.

• Two things at once
• A timer is a remembered tick
• Timeouts

Questions 7 marks in all

1. [1 mark]Why can a program not blink the LED with wait(0.5) while it drives to the zone?

   1. Await(0.5) holds up the whole loop, so the driving stops being updated
   2. Bwait turns the LED off
   3. CThe LED cannot change while the robot moves
   4. Dwait only accepts whole numbers
   Answer: A. In a behaviour nothing waits except the tick. Count ticks instead, and act on every fifth one.

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

   on = False
   changes = 0
   for ticks in range(20):
       if ticks % 5 == 0:
           on = not on
           changes += 1
   print(changes, on)

   Answer:

   4 False

   Ticks 0, 5, 10 and 15 each flip the toggle. Four flips from False brings it back to False.

3. [1 mark]The loop waits 0.1 s a tick. To toggle the LED every half second, every how many ticks should it toggle?

   Answer: 5. Half a second is 5 ticks at ten ticks a second, so ticks % 5 == 0.

4. [1 mark]What does this program print?

   started = None
   for ticks in range(40):
       if started is None and ticks == 20:
           started = ticks
           print("start", ticks)
       elif started is not None and ticks - started >= 10:
           print("stop", ticks)
           started = None

   Answer:

   start 20
   stop 30

   The timer remembers tick 20. ticks - started is the time since then, and it reaches 10 at tick 30, one second later.

5. [1 mark]In started_turning = None, what does None mean?

   1. ANo timer is running
   2. BThe timer has run for 0 ticks
   3. CThe robot has stopped
   4. DThe turn failed
   Answer: A. None says there is no event to count from. When the turn starts, it holds the tick it started on.

6. [1 mark]A search spins while ticks < 30 and gives up if nothing is found. Why is this timeout important?

   1. AWithout it, a search for a tag that is not there would spin forever
   2. BThe camera stops working after 3 seconds
   3. CIt makes the tag easier to see
   4. DIt saves the list of tags
   Answer: A. The most important timer is the one that gives up, so the program can print a message and do something else.

7. [1 mark]What is the advantage of clock() - started_at >= 1.0 over counting ticks?

   1. AIt stays right even if a tick takes longer than planned
   2. BIt works without a loop
   3. CIt makes each tick shorter
   4. DIt stops the robot automatically
   Answer: A. Counting ticks assumes each is exactly 0.1 s. The real clock measures the time that actually passed.

The task: blink and drive

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

# the LED: green
led("green")
# drive forward at 60 for 70 cm, then stop
forward(60, distance=70)
# the LED: off
led("off")

from bugbot import *
connect()
ticks = 0
on = False
while position()[1] < 70:
    forward(60)
    if ticks % 5 == 0:              # every 5 ticks = every half second
        on = not on
        led("green" if on else "off")
    ticks += 1
    wait(0.1)
stop()