Answers

0.2 Numbers and names

Python quick start · Robot club · about 10 min

What this lesson is about

Variables, arithmetic, and driving an exact distance instead of counting seconds.

• A name for a number
• Driving an exact distance

Questions 7 marks in all

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

   speed = 60
   seconds = 1.5
   print("drove at", speed, "for", seconds, "seconds")

   Answer:

   drove at 60 for 1.5 seconds

   Once a variable has a value, its name stands for that value. print shows the values, not the names.

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

   cm_per_second = 18
   seconds = 2
   print("that should be about", cm_per_second * seconds, "cm")

   Answer:

   that should be about 36 cm

   * means times. Python works out 18 times 2 before printing it.

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

   speed = 60
   speed = 25
   print("speed is", speed)

   Answer:

   speed is 25

   A variable holds one value at a time. The second line replaces 60 with 25, so 25 is what gets printed.

4. [1 mark]What does distance=20 mean in forward(50, distance=20)?

   1. ADrive 20 cm, then stop
   2. BDrive for 20 seconds
   3. CDrive at speed 20
   4. DDrive 50 cm, 20 times
   Answer: A. The 50 is the speed and distance=20 is how far, in centimetres. Naming the argument makes it clear which number is which.

5. [1 mark]Do you need a wait() after forward(50, distance=20)?

   1. ANo, the command waits until the robot has gone that far
   2. BYes, wait(20), or the robot never goes anywhere
   3. CYes, otherwise it never stops
   4. DOnly when the speed is over 50
   Answer: A. With a distance, the command itself waits until the move is done. wait() is for commands like forward(50) that return straight away.

6. [1 mark]What does this program print?

   travelled_cm = 85
   print("that is", travelled_cm / 100, "metres")

   Answer:

   that is 0.85 metres

   / divides. There are 100 centimetres in a metre, so 85 cm is 0.85 metres.

7. [1 mark]The robot covers 18 cm every second. How many centimetres does it cover in 2.5 seconds? Give the exact number.

   Answer: 45. Distance is speed times time: 18 times 2.5 is 45 cm.

The task: two legs

from bugbot import *
connect()

first = 40
# your second distance here

forward(50, distance=first)
stop()

from bugbot import *
connect()
first = 40
second = 45
forward(50, distance=first)
right(50, distance=second)
stop()