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5.1 The loop

Behaviours · Robot club · about 15 min

What this lesson is about

Look, decide, act, wait, repeat. One action per tick. Counting ticks.

• Scripts block; behaviours tick
• Why the wait matters
• One action per tick
• Counting ticks

Questions 7 marks in all

1. [1 mark]What does every behaviour do, again and again?

   1. ALook, decide, act, wait, repeat
   2. BDrive, stop, turn, stop
   3. CWait until the task is finished, then end
   4. DRead every sensor once, then act forever
   Answer: A. A behaviour never finishes a move before looking again. Each tick it looks, picks one action and waits a tick.

2. [1 mark]Which call suits a behaviour loop, because it does not block?

   1. Aforward(60)
   2. Bforward(60, distance=20)
   3. Cturn_right(30, angle=90)
   4. Dwait(5)
   Answer: A. forward(60) starts driving and returns at once. With a distance or angle, your program waits until the move is done.

3. [1 mark]A student leaves wait(0.1) out of a while True behaviour loop. What goes wrong?

   1. AThe loop asks the sensors thousands of times with no time passing, and the simulator stops it
   2. BNothing, it just runs faster
   3. CThe robot only does the last tick
   4. DThe robot drives backwards
   Answer: A. The tick needs time to pass so the robot actually moves between looks. The simulator stops the program after 5,000 sensor reads with no time passing. On the real robot the radio would choke.

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

   for tick in [0, 9, 10, 19, 20, 35]:
       print(tick, "green" if tick % 20 < 10 else "off")

   Answer:

   0 green
   9 green
   10 off
   19 off
   20 green
   35 off

   tick % 20 counts 0 to 19 and starts again. Under 10 is green, so it is green for the first second of every two. 35 % 20 is 15, so off.

5. [1 mark]What does this program print?

   def choose(d):
       if d < 15:
           return "back"
       elif d < 30:
           return "turn"
       else:
           return "forward"
   
   for d in [10, 15, 29, 30, 80]:
       print(d, choose(d))

   Answer:

   10 back
   15 turn
   29 turn
   30 forward
   80 forward

   Only the first true test counts. 15 is not under 15, so it falls to the turn. 30 is not under 30, so it goes forward.

6. [1 mark]In one tick a program calls forward(60) and then turn_right(60). What does the robot do?

   1. ATurns right: only the last command counts
   2. BDrives forward: only the first command counts
   3. CDrives forward and turns at once
   4. DStops, because the commands clash
   Answer: A. Each command replaces the one before. That is why one tick should choose exactly one action with if / elif / else.

7. [1 mark]A behaviour loop waits 0.1 s each tick. How many ticks is five seconds?

   Answer: 50. Ten ticks a second, so five seconds is 50 ticks.

The task: wander

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

# drive forward at 60 (keeps going until the next command)
forward(60)
# pause 5 s (the robot keeps doing what it was told)
wait(5)
# all motors off
stop()

from bugbot import *
connect()
for tick in range(190):
    if distance() < 25:
        turn_right(60)          # something close: turn away
    else:
        forward(60)             # clear: go
    wait(0.1)
stop()