B2 Practical Scenario Book

Coverage map

All scenarios, prices and data in this book are fictional and written for this course. Mark allocations follow IB conventions but this is a teaching resource, not past-paper material.

(a) State the most appropriate data type for each of these, and give one reason for each choice. [6]  ~6 min

(b) Explain why input() alone cannot be used to collect the number of coffees, and state what must be done to the value. [3]  ~4 min

(c) Complete the skeleton below so it prices one order and applies both discount rules correctly. [6]  ~10 min

COFFEE_PRICE   = 3.50
STAFF_DISCOUNT = 0.20
MEETING_SIZE   = 6
MEETING_OFF    = 0.50

qty      = int(input("How many coffees? "))
is_staff = input("Staff? (y/n) ").lower() == "y"

unit = COFFEE_PRICE
if _______________________ :          # meeting discount applies?
    unit = _______________________

total = _______________________       # before the staff discount

if _______________________ :          # staff?
    total = _______________________

print("Total: $" + str(round(total, 2)))

(d) Deduce the total your completed code produces for 8 coffees bought by a staff member. Show your working. [3]  ~4 min

(e) A student writes the price band code below. Deduce what it prints for a total of 25.00, and explain why. [4]  ~5 min

if total > 5:
    print("Small order")
elif total > 15:
    print("Medium order")
elif total > 20:
    print("Large order - free muffin!")

(f) Rewrite the code from (e) so it works correctly. [3]  ~4 min

(g) The café adds a rule: an order qualifies for free delivery to the staff room if it is either over $20 or placed by a staff member, but only when there are at least 3 items. Write the condition as a single Python if, using brackets to make the precedence explicit. [3]  ~4 min

(h) Exam-style. Explain why the prices should be stored as named constants rather than typed directly into the calculations. [2]  ~3 min

(a) Explain why a while loop is required here rather than a for loop. [3]  ~4 min

(b) State what a sentinel value is, and explain why -1 is a suitable choice here but 0 would not be. [3]  ~4 min

(c) Complete the skeleton so it accumulates the morning's takings and counts the customers. [6]  ~10 min

COFFEE_PRICE = 3.50
SENTINEL     = -1

takings   = _______________          # initialise the accumulator
customers = _______________          # initialise the counter

qty = int(input("Coffees (-1 to finish): "))

while _______________________ :
    takings   = _______________________
    customers = _______________________
    qty = int(input("Coffees (-1 to finish): "))     # why is this here?

print("Customers:", customers)
print("Takings: $" + str(round(takings, 2)))

(d) Explain why the second input() must be the last statement inside the loop, and state what would happen if it were the first. [3]  ~4 min

(e) Trace the completed loop for the inputs 2, 3, 1, −1. Complete every row. [6]  ~9 min

(f) State the two lines printed at the end. [2]  ~2 min

(g) This loop never ends. Identify the fault, explain why it happens, and write the corrected code. [4]  ~5 min

i = 0
while i < 5:
    print("Order", i)

(h) The café wants a receipt line printed for every individual coffee in every order (so an order of 2 prints two lines). Write the nested loop that does this for 3 orders, and state how many lines the inner loop body runs in total if the three orders are 2, 1 and 4 coffees. [4]  ~7 min

(i) Exam-style. Distinguish between a count-controlled and a condition-controlled loop, giving one example of each from the café system. [3]  ~4 min

(a) Explain two problems caused by the discount calculation being copied in three places. [4]  ~5 min

(b) Complete the function below. It must return the cost of an order, applying the meeting discount and the staff discount from Scenario 1. [6]  ~10 min

COFFEE_PRICE   = 3.50
STAFF_DISCOUNT = 0.20
MEETING_SIZE   = 6
MEETING_OFF    = 0.50

def order_cost(qty, is_staff=False):
    """Return the cost of qty coffees, applying any discounts."""
    if qty _______________ 0:                 # guard: nothing ordered
        return _______________

    unit = COFFEE_PRICE
    if _______________________ :
        unit = _______________________

    total = _______________________

    if is_staff:
        total = _______________________

    _______________ round(total, 2)           # hand the value back

print(order_cost(2))
print(order_cost(8, True))

(c) Distinguish between a parameter and an argument, using order_cost to illustrate your answer. [3]  ~4 min

(d) State the purpose of is_staff=False in the definition, and state what order_cost(4) therefore calculates. [2]  ~3 min

(e) A student rewrites the function to print the total instead of returning it. Explain two things this makes impossible. [4]  ~5 min

(f) Deduce the output of this program and explain it with reference to scope. [4]  ~6 min

takings = 100

def add_sale(amount):
    takings = amount
    print("inside:", takings)

add_sale(20)
print("outside:", takings)

(g) Rewrite add_sale so the caller's running total is correctly updated, without using a global variable. [3]  ~5 min

(h) The supervisor asks for a recursive function that prints a countdown of the last 5 minutes before closing. Complete it, and state what would happen if the base case were removed. [4]  ~6 min

def countdown(mins):
    if _______________________ :          # base case
        print("Closed.")
        _______________
    print(mins, "minutes left")
    countdown(_______________)             # recursive case

countdown(5)

(i) Exam-style. The café system is now split into order_cost, take_payment and print_receipt. Explain two benefits of this structure for a second programmer joining the project. [4]  ~6 min

ITEMS = ["coffee", "tea", "juice", "muffin"]
PRICES = {"coffee": 3.50, "tea": 2.00, "juice": 2.50, "muffin": 1.75}

# rows = Mon, Tue, Wed, Thu, Fri   columns = coffee, tea, juice, muffin
sales = [[24, 8, 5, 12],
         [31, 6, 9, 15],
         [19, 11, 4, 7],
         [28, 9, 7, 18],
         [35, 4, 11, 22]]

(a) State the value of sales[1][3], len(sales) and len(sales[0]), and say what each represents. [3]  ~4 min

(b) Write code that prints the total items sold each day (one line per day). [3]  ~5 min

(c) Complete the skeleton that totals each item across the whole week. Explain in one sentence why this needs the loops the other way round from part (b). [5]  ~8 min

for col in range(_______________________):        # one pass per ITEM
    item_total = 0
    for row in range(_______________________):    # step down the DAYS
        item_total = item_total + _______________________
    print(ITEMS[col], item_total)

(d) Write code that calculates the café's total revenue for the week, using both sales and PRICES. [4]  ~7 min

(e) Write code that finds the single busiest day (by items sold) and prints its index and total. [4]  ~7 min

(f) A student writes best = 0 before searching for the busiest day, then compares totals. Explain why this happens to work here but is a bad habit, and state the safer way to initialise. [3]  ~4 min

(g) Write a single line that turns any of " Amina ", "AMINA" or "amina" into the same comparable value, and state which two string methods you used. [3]  ~4 min

(h) The order log holds lines like "Amina,coffee,2". Write code that splits one such line into its three parts and prints the cost of that order using PRICES. [4]  ~6 min

(i) Explain why PRICES is a dictionary rather than a list, giving two reasons. [3]  ~4 min

(j) Exam-style. A student writes backup = sales then edits backup[0][0], and finds sales has changed too. Explain why, and state the correction. [2]  ~3 min

(a) Name the exception raised by each of the four incidents above. [4]  ~5 min

(b) Complete the robust input routine below. It must keep asking until a valid quantity (0–20) is entered, and must never crash. [6]  ~10 min

MAX_QTY = 20

def get_quantity():
    while True:
        _______________:                              # start protecting
            qty = int(input("How many? "))
        _______________ ValueError:                   # not a whole number
            print("Please enter a whole number.")
            continue
        if _______________________ :                  # range check
            print("Must be between 0 and", MAX_QTY)
            continue
        _______________ qty                           # valid - hand it back

(c) Explain why a bare except: would be poor practice here. [3]  ~4 min

(d) Name the validation check that would prevent each incident, and state the check's purpose. [4]  ~5 min

(e) Identify the correct ADT for each new feature, state its discipline, and justify the choice from the café's wording. [4]  ~6 min

(f) Complete the queue and stack operations. [5]  ~8 min

order_queue = []          # customers waiting to be served
undo_stack  = []          # items added to the current order

def join_queue(name):
    order_queue._______________(name)          # enqueue at the rear

def serve_next():
    if _______________________ :               # guard against underflow
        return None
    return order_queue._______________         # dequeue from the FRONT

def add_item(item):
    undo_stack._______________(item)

def undo_last():
    if len(undo_stack) == 0:
        return None
    return undo_stack._______________          # remove the most recent

(g) Amina, Ben and Chen join the queue in that order; one customer is served; Dina joins. State who is served next and who is at the rear. [2]  ~2 min

(h) Define underflow, and explain why the guard in serve_next is necessary. [2]  ~3 min

(i) Exam-style. Distinguish between validation and verification, and explain which one would not have prevented a duty student typing 3 coffees when the customer asked for 2. [2]  ~3 min

Amina,coffee,2,7.00
Ben,muffin,1,1.75
Chen,tea,3,6.00

(a) State the Big O complexity of each, and justify each in one clause. [4]  ~5 min

(b) Calculate the maximum number of comparisons a binary search needs on 12 000 sorted names, and show your reasoning. [3]  ~4 min

(c) Complete the binary search. [5]  ~9 min

def binary_search(names, target):
    low  = 0
    high = _______________________

    while low _______________ high:
        mid = _______________________
        if names[mid] == target:
            return mid
        elif names[mid] < target:
            low  = _______________________
        else:
            high = _______________________
    return -1

(d) Trace the binary search for "Nadia" in the sorted list below. Complete every row. [5]  ~8 min

index:  0        1      2       3       4       5      6       7
name:   Amina    Ben    Chen    Dina    Eve     Nadia  Omar    Sara

(e) Explain why binary search would give the wrong answer if the names were not sorted. [2]  ~3 min

(f) The supervisor asks why you do not "just always use binary search". Explain when a linear search is the better choice for the café. [3]  ~4 min

(g) Complete one full pass of a bubble sort on the daily takings [41, 18, 37, 22], showing each comparison and whether a swap occurred. [4]  ~6 min

(h) State which value is guaranteed to be in its final position after that pass, and explain why. [2]  ~2 min

(i) Complete the file-reading routine. It must survive a missing file and skip any malformed line. [6]  ~10 min

def load_orders(filename):
    """Return a list of (name, item, qty, cost) tuples."""
    orders = []
    try:
        _______________ open(filename, "_______") as f:     # read mode
            for line in f:
                line = line._______________()               # remove newline
                if line == "":
                    continue
                parts = line.split("_______")
                if len(parts) != 4:
                    print("Malformed line skipped:", line)
                    continue
                orders.append((parts[0], parts[1],
                               _______________(parts[2]),   # qty as a number
                               float(parts[3])))
    _______________ FileNotFoundError:
        print(filename, "not found - starting with no data")
    return orders

(j) Explain why "a" must be used rather than "w" when adding a new order to orders.txt. [2]  ~2 min