Answers

0.4 Deciding

Python quick start · Robot club · about 12 min

What this lesson is about

if, else, comparisons, and a robot that looks with distance() before it moves.

• The distance sensor
• if and else
• Stopping at the right moment

Questions 7 marks in all

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

   gap = 25
   if gap > 30:
       print("plenty of room")
   else:
       print("too close")

   Answer:

   too close

   25 is not more than 30, so the test is false and the else lines run instead.

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

   gap = 30
   if gap > 30:
       print("go")
   else:
       print("stop")

   Answer:

   stop

   30 is not more than 30, it is equal. > is false when the two are the same, so this prints stop. Use >= to include 30.

3. [1 mark]Which line asks whether gap is 15?

   1. Aif gap == 15:
   2. Bif gap = 15:
   3. Cif gap is equal 15:
   4. Dif 15:
   Answer: A. Two equals signs ask a question. One equals sign gives a name a value, and Python will not accept it in an if.

4. [1 mark]What does this program make the robot do?

   while distance() > 15:
       forward(40)
       wait(0.1)
   stop()

   1. ADrive forward, looking as it goes, and stop once the gap is 15 cm or less
   2. BDrive exactly 15 cm and stop
   3. CDrive backwards away from the wall
   4. DDrive until it touches the wall
   Answer: A. The while looks at distance() before every repeat. As soon as the gap is not more than 15, the loop ends and stop() runs.

5. [1 mark]The same program runs with the robot already 10 cm from the wall. What happens?

   while distance() > 15:
       forward(40)
       wait(0.1)
   stop()

   1. AThe loop never runs, so the robot stops straight away without moving
   2. BIt drives into the wall
   3. CIt backs up to 15 cm
   4. DPython gives an error
   Answer: A. The test distance() > 15 is false the very first time, so the indented lines are skipped and the program goes straight to stop().

6. [1 mark]gap is 20. Which of these tests are true?

   Tick every answer that is true.

   1. Agap > 15
   2. Bgap >= 20
   3. Cgap != 20
   4. Dgap < 20
   5. Egap == 20
   Answer: A, B, E. 20 is more than 15, and it is equal to 20, so >= and == are true. It is not less than 20, and != asks whether it is different, which it is not.

7. [1 mark]Which two symbols together mean "is not" in Python?

   Answer: !=. != is true when the two sides are different, so gap != 0 means the gap is anything except 0.

The task: stop at the wall

from bugbot import *
connect()

while distance() > 40:
    forward(40)
    wait(0.1)

stop()
print("gap", distance())

from bugbot import *
connect()
while distance() > 15:
    forward(40)
    wait(0.1)
stop()
print('gap', distance())