Debugging Examples in CS Thinking
Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Debugging.
This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in CS Thinking.
Concept Recap
The systematic process of finding, diagnosing, and correcting errors (bugs) in a program. Debugging involves reproducing the problem, isolating its cause through testing and inspection, applying a targeted fix, and verifying the fix resolves the issue without introducing new problems.
Detective work—observe the wrong output, form a hypothesis, test it, then fix what's wrong.
Read the full concept explanation →How to Use These Examples
- Read the first worked example with the solution open so the structure is clear.
- Try the practice problems before revealing each solution.
- Use the related concepts and background knowledge badges if you feel stuck.
What to Focus On
Core idea: Debugging is systematic: reproduce the bug, isolate the cause, apply a fix, then verify it works.
Common stuck point: The bug might not be where the error appears—trace backward.
Sense of Study hint: When debugging, follow these steps: first reproduce the bug reliably with a specific input. Then narrow down where the problem occurs by adding print statements or using a debugger to inspect variable values. Once you find the faulty line, fix it and test with the original failing input plus other cases.
Common Mistakes to Watch For
Before you work through the examples, skim the mistake guide so you know which shortcuts and sign errors to avoid.
Worked Examples
Example 1
mediumThis code should print numbers 1 to 5 but has a bug: SET i = 1. WHILE i < 5: OUTPUT i. i = i + 1. Find and fix the bug.
Answer
Change `i < 5` to `i <= 5`. Off-by-one error.
First step
1
Step 1: Trace: i=1 (output 1), i=2 (output 2), i=3 (output 3), i=4 (output 4), i=5 (5 < 5 is false, loop ends).
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Example 2
mediumThis code calculates an average but gives the wrong answer: SET total = 0. FOR each num in [10, 20, 30]: total = total + num. SET average = total / 2. OUTPUT average. The input has 3 numbers. Find the bug.
Example 3
mediumThe code `SET i = 0; WHILE i <= 5: OUTPUT i` prints how many numbers?
Example 4
medium`if score >= 90: grade = 'A' elif score >= 80: grade = 'B' elif score >= 70: grade = 'A'`. A student scores 75 and gets 'A'. Find the bug.
Example 5
mediumA bug is somewhere in commits. Using bisection (halve each time), what is the worst-case number of test runs?
Example 6
mediumThis computes a maximum: `max = nums[0]; FOR i = 1 TO LENGTH(nums)-1: IF nums[i] > max THEN max = nums[i]`. For nums = [4, 9, 2, 7], what does it return?
Example 7
hardThis binary search bug: `low = 0; high = LENGTH(arr)-1; WHILE low <= high: mid = (low + high) / 2; ...`. In integer division on negatives, what may go wrong if low+high is negative due to signed underflow?
Example 8
hardTrace this: `a=[1,2,3]; b=a; b.append(4); print(len(a))`. Output?
Example 9
challengeA buggy function returns `nums[i+1]` where nums has length and i runs up to . What error happens at the last iteration and how do you fix the index bound?
Practice Problems
Try these problems on your own first, then open the solution to compare your method.
Example 1
mediumThis code should output the largest number in [3, 7, 2, 9, 4] but outputs 3: SET largest = nums[0]. FOR i = 0 TO LENGTH(nums)-1: IF nums[i] > largest THEN largest = nums[i]. OUTPUT largest. Trace and verify if it is correct, or find the bug.
Example 2
mediumA program to sum the numbers 1 through 5 uses `SET total = 1; FOR i = 1 TO 5: total = total + i`. It outputs 16 instead of 15. Identify the bug and fix it.
Example 3
easyWhat is the FIRST step of systematic debugging?
Example 4
easyA program crashes only on empty input. What kind of value triggers many bugs and should be tested?
Example 5
easy`if x = 5:` causes an error in many languages. What's the bug?
Example 6
easyAfter fixing a bug, what must you always do before declaring it done?
Example 7
easyA function returns the wrong answer. To isolate the cause, what tool prints intermediate values?
Example 8
easyA bug keeps coming back in different forms. What did the previous fix likely address?
Example 9
easyChanging many things at once then testing makes debugging harder. What's the better approach?
Example 10
easyA stack trace points to line 42. What does that tell you?
Example 11
medium`def avg(nums): return sum(nums) / len(nums)`. It crashes on `avg([])`. What is the bug and edge case?
Example 12
mediumA loop `i = 0; while i < 5: print(i)` never stops. What's missing?
Example 13
mediumOutput is right for input 5 but wrong for input 0. Which test category caught this?
Example 14
medium`def f(x): return x * 2` is expected to give 6 for input 3 but gives `33`. What does the actual behavior reveal about `x`?
Example 15
mediumBisection debugging: a bug is somewhere in 16 commits. Roughly how many test runs (worst case) to isolate it by halving?
Example 16
mediumTwo functions both look correct but together produce a wrong result. What kind of bug is this likely to be?
Example 17
mediumA bug only appears sometimes. What property makes it hard to debug?
Example 18
challengeA function works for n=1,2,3 but fails for n=4 with a wrong sum. You suspect an off-by-one. In `total = 0; for i in 1..n: total += i` (range excludes n), what is computed for n=4 vs the intended 1+2+3+4?
Example 19
challengeCode reads a global `config` and behaves differently in tests vs production. What hidden factor is causing the inconsistency?
Example 20
challengeYou fix bug A and immediately bug B appears in a previously-working feature. What is this called and what prevents it?
Example 21
mediumA test passes locally but fails on the build server. Changing one variable at a time, you find it's the OS file path separator. What debugging principle found it?
Example 22
medium`x = [1,2,3]; for i in x: x.append(i)`. Why is this dangerous?
Example 23
easyA code uses `if name = 'Sam':` and errors. What's wrong?
Example 24
easyAfter a fix, what must you do before declaring the bug closed?
Example 25
easyWhat does adding a `print(x)` statement help with?
Example 26
easyA function works for input but crashes for input . What category of test catches ?
Example 27
medium`total = 0; FOR i = 1 TO 4: total = total + i; OUTPUT total`. What does it print?
Example 28
mediumThis code should sum 1..n: `total = 0; FOR i = 1 TO n: total = total + i`. For n=10 it should give 55. It returns 45. What's wrong?
Example 29
medium`def avg(nums): return sum(nums)/len(nums)`. Calling `avg([])` errors. What's the edge case and a fix?
Example 30
mediumA bug only happens on the build server, not your laptop. What general kind of factor often causes this?
Example 31
medium`x = '3'; y = 2; print(x + y)` errors. What's the bug, and how do you fix it for numeric addition?
Example 32
medium`for item in items: items.remove(item)`. Why is this dangerous?
Example 33
mediumA test passes locally but fails in CI. Which debugging principle helps isolate the cause?
Example 34
hardA function returns the WRONG answer for n=4, expected but got . The body is `total=0; for i=1..n step 1: total+=i` where the range is exclusive of n. What did it sum, and what is the fix?
Example 35
hardA regression appears: fixing bug A broke working feature B. What kind of automated test would have caught it earliest?
Example 36
hardA function reads a global config and behaves differently between test and prod. What is the hidden coupling?
Example 37
challengeA bug appears 1 in 100 runs and never with the debugger attached. What general category is this, and what tool helps?
Related Concepts
Background Knowledge
These ideas may be useful before you work through the harder examples.
algorithm