CSC 1001 — Lab 8: Tracing and Fixing Buggy Programs

Intro

You will work with a partner to trace through a small buggy program, agree on what is broken, and figure out why. Then you will each individually attempt a fix and compare your solutions. This lab builds the skill of reading and reasoning about code carefully before changing anything — the same habit you will lean on with any tool, including AI coding assistants, later in the course.

Submit: your Lab 8 Worksheet and a notebook or script (Lab8.ipynb) with your fixed program.

Understand (with your partner)

• Pick one of the 7 programs in the appendix below.
• Trace through the program by hand, line by line, tracking what each variable holds as the program runs. Record your trace on your Lab 8 Worksheet.
• Agree with your partner on what the problem(s) is/are, and note it on your worksheet, before moving on.
• If you can't find the problem right away, step back and ask:
  • What is the big picture goal of this program?
  • What is the big picture goal of this specific function?
  • What should each variable represent at each point in the program?
  • Where does what actually happens first stop matching what you expected?
• If you still can't find the problem after several traces, go ahead and run the program. Try to get as far as you can on paper first — tracing by hand is one of the most powerful debugging skills you can build, and it works even when you don't have a computer in front of you.

Do (individually)

• Working on your own now, look back over your trace.
• If you can see the fix immediately, make the change in Lab8.ipynb.
• If you can't fix it immediately, step back through these checkpoints:
  • Program: does the overall structure match what the program is supposed to do?
  • Function: does each function do exactly one clear job, and does it do that job correctly?
  • Conditionals: are the comparisons checking the right thing, in the right direction?
  • Variables: does every variable hold what you think it holds at each step?
  • Operators: is each operator (+, -, *, /, ==, <, >=, and, or, ...) the one that is actually needed here?
• Once you know what the problem is, the fix itself is usually a small, close-by change — you shouldn't need to rewrite the whole program.
• Use the trace table on your Lab 8 Worksheet to keep track of your work, then implement your fix in Lab8.ipynb.

Communicate (with your partner)

• Compare your fix with your partner's fix.
• Discuss, and record your answers on your Lab 8 Worksheet:
  • Did you both find the same problem? If not, why might you have looked in different places?
  • Did you fix it the same way? Is one fix more complete, or more general, than the other?
  • Was there a moment where stepping back to the big picture helped you find something a closer read had missed?

Repeat Understand/Do/Communicate

Complete the same steps for at least 3 total buggy programs.

Appendix: Buggy Programs

Choose one program with your partner. Each program is short on purpose — the goal is careful reading, not reading a lot of code.

Program 1: Letter Grade Assigner

def letter_grade(score: int):
    if score >= 90
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    else:
        return "F"

print(letter_grade(85))

Program 2: Order Total Calculator

def total_cost(price: float, quantity: int):
    subtotal: float = price * quantity
        tax: float = subtotal * 0.05
    return subtotal + tax

print(total_cost(19.99, 3))

Program 3: Test Score Averager

def average(numbers):
    total: int = 0
    for n in numbers:
        total: int = total + n
    return total / len(numbers)

scores: list[int] = []
print(average(scores))

Program 4: Greeting Builder

def greet(name: str, age: int):
    message = "Hello " + name + ", you are " + age + " years old."
    return message

print(greet("Sam", 19))

Program 5: Even Number Counter

def count_evens(numbers: list[int]):
    count: int = 0
    for i in range(1, len(numbers)):
        if numbers[i] % 2 == 0:
            count = count + 1
    return count

print(count_evens([2, 4, 6, 8, 10]))

Program 6: Passing Score Checker

def is_passing(score: int):
    if score < 60:
        return True
    else:
        return False

print(is_passing(75))

Program 7: Shipping Fee Estimator

def shipping_fee(weight: int, distance: int):
    base_fee: int = 5
    fee: float = base_fee + weight * 0.5
    return fee

print(shipping_fee(10, 200))