Answers

F4.6 Project: the patrol

Functions and structured code · GCSE · about 25 min

What this lesson is about

A complete program built a function at a time: patrol to the wall and come home.

• The brief
• Decompose it
• Piece 1: one step
• Piece 2: the patrol
• Piece 3: coming home
• Check the whole program
• Where next

Questions 5 marks in all

1. [1 mark]Why test step(n) on its own before writing the loop?

   1. AMistakes are easiest to find while the program is small
   2. BPython needs functions to be run once first
   3. CIt charges the battery
   4. DThe loop will not work otherwise
   Answer: A. Building a program a piece at a time means each piece is checked while it is still short.

2. [1 mark]What does a name written in capitals, like STOP_AT = 30, tell someone reading the program?

   1. AIt is set once and not changed while the program runs
   2. BPython will not let it change
   3. CIt is a function
   4. DIt is printed automatically
   Answer: A. Capitals are a convention for values set once. Python does not enforce it; it is a message to people.

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

   n = 1
   while n <= 3:
       print(f"step {n}")
       n = n + 1
   print("stopped after", n - 1, "steps")

   Answer:

   step 1
   step 2
   step 3
   stopped after 3 steps

   The counter goes up after every step, so when the loop ends n is one more than the number of steps.

4. [1 mark]In the patrol, why does patrol(stop_at) return the number of steps rather than print it?

   1. Ago_home needs the value to drive back the same distance
   2. BPrinting is not allowed in functions
   3. Creturn is faster
   4. DSo nothing appears on screen
   Answer: A. Return a value when another part of the program needs to use it.

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

   def patrol(readings):
       n = 0
       for d in readings:
           if d <= 30:
               break
           n = n + 1
       return n
   
   print(patrol([60, 50, 40, 30, 20]))

   Answer:

   3

   It counts steps until a reading is 30 or less: 60, 50 and 40 count, then it stops.

The task: patrol

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

def step(n):
    forward(50, distance=10)
    print(f"step {n}: {distance()} cm")

# the loop, then the ending

from bugbot import *
connect()
def step(n):
    forward(50, distance=10)
    print(f'step {n}: {distance()} cm')
n = 1
while distance() > 30:
    step(n)
    n = n + 1
print('arrived')
led('green')