PCEP Control Flow, Loops, Lists and Logic Practice Question
This PCEP practice question tests your understanding of control flow, loops, lists and logic. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A program uses a while loop to find the largest number in a list until a negative number is encountered. What is wrong with this code?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The loop should continue even when number is negative (skip negatives).
Option C is correct because the while loop condition `numbers[i] >= 0` causes the loop to terminate as soon as it encounters a negative number (-1), so it never processes the number 5 at the end of the list. The intended behavior is to skip negative numbers and continue iterating through the entire list to find the largest number among all non-negative values. To fix this, the loop should use a condition that checks the index against the list length (e.g., `i < len(numbers)`) and then skip negative numbers with an `if` statement inside the loop.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✗
The loop should start with max_num = None.
Why it's wrong here
Does not fix early termination.
✗
The loop should check all numbers until end of list.
Why it's wrong here
This would also fix it, but the condition needs to not break on negative; however, 'continue' is more precise. Option B is the best fix.
✓
The loop should continue even when number is negative (skip negatives).
Why this is correct
This would allow processing all numbers, not stopping at -1.
Related concept
Read the scenario before looking for a memorised answer.
✗
The loop should use a for loop instead.
Why it's wrong here
Does not address the logic error.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Python Institute often tests the misconception that a while loop's condition should mirror the data filter (e.g., 'continue while number is non-negative'), when in reality the condition should control the iteration range (e.g., index within bounds) and the filter should be an inner `if` statement.
Detailed technical explanation
How to think about this question
The while loop's condition `numbers[i] >= 0` is evaluated before each iteration, so when `i` reaches index 3 (value -1), the condition fails and the loop exits immediately, leaving the last element (5) unexamined. In Python, list indexing with a negative value does not cause an error here because the loop stops before accessing `numbers[4]`. This pattern is common in data-stream processing where a sentinel value (like -1) signals the end of input, but the code incorrectly uses the sentinel as a loop-termination condition instead of a skip condition.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Control Flow, Loops, Lists and Logic — This question tests Control Flow, Loops, Lists and Logic — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The loop should continue even when number is negative (skip negatives). — Option C is correct because the while loop condition `numbers[i] >= 0` causes the loop to terminate as soon as it encounters a negative number (-1), so it never processes the number 5 at the end of the list. The intended behavior is to skip negative numbers and continue iterating through the entire list to find the largest number among all non-negative values. To fix this, the loop should use a condition that checks the index against the list length (e.g., `i < len(numbers)`) and then skip negative numbers with an `if` statement inside the loop.
What should I do if I get this PCEP question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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