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F2.4 Nested selection and match

Decisions and loops · GCSE · OCR J277 2.2.1, AQA 8525 3.2.2, Edexcel 1CP2 6.2.2 · about 15 min

What this lesson is about

Decisions inside decisions, and choosing by exact value with a case statement.

• A decision inside a decision
• Nested or joined?
• Tracing a nested decision
• match: choosing by exact value

Questions 6 marks in all

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

   d = 60
   b = 10
   if d < 30:
       if b < 20:
           print("stuck and flat")
       else:
           print("stuck")
   else:
       if b < 20:
           print("free but flat")
       else:
           print("free")

   Answer:

   free but flat

   d < 30 is False, so the outer else runs. Inside it, b < 20 is True.

2. [1 mark]Which else does an else belong to in nested selection?

   1. AThe if it lines up with
   2. BAlways the first if in the program
   3. CAlways the nearest if above it, whatever the indentation
   4. DThe last if in the program
   Answer: A. Indentation decides: an else pairs with the if at the same indentation.

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

   command = "left"
   match command:
       case "forward":
           print("f")
       case "left" | "right":
           print("sideways")
       case _:
           print("unknown")

   Answer:

   sideways

   The first case that matches runs. | means or, so "left" matches the second case.

4. [1 mark]What does case _: do in a match?

   1. AMatches any value not matched above, like else
   2. BMatches an empty string
   3. CEnds the match
   4. DCauses an error
   Answer: A. The underscore matches anything, so it catches every other value. Put it last.

5. [1 mark]OCR's Exam Reference Language writes a case statement with which keyword? (one word)

   Answer: switch. switch ... case ... default ... endswitch. Python's version is match.

6. [1 mark]Which is the same as if a > 30: if b > 50: print("go") (with no else parts)?

   1. Aif a > 30 and b > 50: print("go")
   2. Bif a > 30 or b > 50: print("go")
   3. Cif not a > 30: print("go")
   4. Dif a > 30: print("go")
   Answer: A. Both conditions must be True to reach the print, which is exactly what and says.

The task: command mode

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

command = input("Which way? ")
far = float(input("How far? "))

from bugbot import *
connect()
command = input("Which way? ")
far = float(input("How far? "))
match command:
    case "forward":
        forward(50, distance=far)
    case "back":
        backward(50, distance=far)
    case "left":
        left(50, distance=far)
    case "right":
        right(50, distance=far)
    case _:
        print("unknown")