Answers

F2.3 else, elif and Boolean operators

Decisions and loops · GCSE · OCR J277 2.2.1, AQA 8525 3.2.5, Edexcel 1CP2 1.2.3 · about 15 min

What this lesson is about

Choosing between outcomes, and joining conditions with and, or and not.

• else: what to do otherwise
• elif: several choices
• Joining conditions: and, or, not
• Checking a range

Questions 8 marks in all

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

   d = 30
   if d < 20:
       print("stop")
   elif d < 40:
       print("careful")
   else:
       print("go")

   Answer:

   careful

   Python checks from the top and runs the first block whose question is True: 30 < 20 is False, 30 < 40 is True.

2. [1 mark]With an if and an else, how many of the two blocks run?

   1. AExactly one
   2. BBoth
   3. CNeither
   4. DIt depends on the program
   Answer: A. The if block when the question is True, the else block when it is False. Never both, never neither.

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

   print(True and False)
   print(True or False)
   print(not True)

   Answer:

   False
   True
   False

   and needs both sides True, or needs at least one, and not flips the answer.

4. [1 mark]The traffic light checks d < 40 first and d < 20 second, with elif. What goes wrong when d is 10?

   1. AIt says careful instead of stop: the first True question wins
   2. BNothing goes wrong
   3. CIt says both
   4. DAn error
   Answer: A. 10 < 40 is True, so that block runs and the rest are skipped. With elif, the order of the questions matters.

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

   d = 50
   b = 10
   if d > 30 and b > 20:
       print("drive")
   else:
       print("stay")

   Answer:

   stay

   d > 30 is True but b > 20 is False, and and needs both.

6. [1 mark]What is elif short for? (two words)

   Answer: else if. elif means else if: checked only when the questions above it were False.

7. [1 mark]In a truth table, when is A OR B False?

   1. AOnly when A and B are both False
   2. BWhen either is False
   3. CWhen both are True
   4. DNever
   Answer: A. OR needs at least one side True, so it is False only when both sides are False.

8. [1 mark]What does this program print?

   d = 25
   if d >= 20 and d <= 40:
       print("in the zone")
   else:
       print("outside")

   Answer:

   in the zone

   25 is at least 20 and at most 40, so both sides are True.

The task: traffic light

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

forward(50, distance=15)
d = distance()
if d < 20:
    led("red")
    print("stop")

from bugbot import *
connect()
forward(50, distance=15)
d = distance()
if d < 20:
    led('red')
    print('stop')
elif d < 40:
    led('orange')
    print('careful')
else:
    led('green')
    print('go')