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A15.4 Pseudocode in the exam languages

Exam preparation · A level · OCR H446 2.2.1, AQA 7517 4.4.1.2, Eduqas A500QS 1.8 · about 45 min

BugBotLab

What this lesson is about

OCR's exam reference language and AQA's pseudo-code side by side with Python, the traps in translating, and a stack class that brings the robot home.

Questions 5 marks in all

  1. [1 mark]How many times does the AQA pseudo-code loop FOR i ← 1 TO 10 run?

    1. A10
    2. B9
    3. C11
    4. DIt depends on the language
    Answer: A. Both ends are included, so i takes the values 1 to 10. In Python this is range(1, 11).

  2. [1 mark]Which Python matches the OCR loop do ... until reading < 30?

    1. Awhile True: with the body, then if reading < 30: break
    2. Bwhile reading < 30: with the body
    3. Cfor reading in range(30): with the body
    4. Dif reading < 30: with the body
    Answer: A. A post-condition loop runs the body at least once and stops when its condition becomes true.

  3. [1 mark]In AQA pseudo-code, what does x ← 3 do?

    1. AAssigns 3 to x
    2. BChecks whether x equals 3
    3. COutputs 3
    4. DMoves x three places left
    Answer: A. AQA uses the arrow for assignment and = for comparison.

  4. [1 mark]In OCR's exam reference language, what is the name of a class's constructor?

    Answer: new. OCR writes public procedure new(...), called with the keyword new, as in myStack = new Stack().

  5. [1 mark]This is a stack from the exam reference language, translated. What does it print?

    items = [None] * 5
top = -1
for move in ["forward", "right", "left"]:
    top = top + 1
    items[top] = move
while top != -1:
    print(items[top])
    top = top - 1
    Answer:
    left
right
forward

    The pointer version still behaves as a stack: the last item pushed is the first popped.

The task: there and back with a stack

Translate the Stack class above into Python, then use it to bring the robot home. Write class Stack with: - __init__(self): an empty stack; - push(self, item): puts item on the top; - pop(self): removes and returns the top item; - is_empty(self): returns True if there are no items, otherwise False. route is a list of moves in order. Each move is a tuple (direction, cm): direction is one of "forward", "backward", "left" or "right", and cm is a whole number of centimetres. Drive each move in order at speed 50, pushing it onto a stack after it is driven. The robot then reaches the charger: play a note and print arrived. To come home, loop while not <stack>.is_empty(): pop a move, print undo <direction> <cm>, and drive the opposite direction (forward and backward are opposites, as are left and right) the same distance at speed 50. After the loop print home. Do not reverse the list yourself: the stack must do it.

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

route = [("forward", 30), ("right", 20), ("forward", 15), ("left", 10)]

The hint students can ask for: Translate the pseudocode one method at a time, keeping its structure. Drive each move of the route and push it as you go. To come back, pop a move, drive the opposite way the same distance, and repeat until the stack is empty: the last move out is the first undone.

A solution

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

class Stack:
    def __init__(self):
        self.__items = []

    def push(self, item):
        self.__items.append(item)

    def pop(self):
        if self.is_empty():
            raise IndexError("pop from an empty stack")
        return self.__items.pop()

    def is_empty(self):
        return len(self.__items) == 0

OPPOSITE = {"forward": "backward", "backward": "forward", "left": "right", "right": "left"}
MOVES = {"forward": forward, "backward": backward, "left": left, "right": right}
route = [("forward", 30), ("right", 20), ("forward", 15), ("left", 10)]

done = Stack()
for direction, cm in route:
    MOVES[direction](50, distance=cm)
    done.push((direction, cm))
tone(880, 0.3)
print("arrived")
while not done.is_empty():
    direction, cm = done.pop()
    print("undo", direction, cm)
    MOVES[OPPOSITE[direction]](50, distance=cm)
print("home")

Any program that meets the task's checks is marked correct in the simulator; this is one way, not the only way.