These questions cover the whole problem-solving explanations module: the purpose of a variable, justified test cases, described corrections, traces linked to output and minimal examples. They are original and ordered from easier to harder.
Write your answer in full sentences first. For traces, draw the table.
Then open the answer and compare. The restricted pseudocode trace trainer and the safe Python reasoning sandbox can check your values.
Questions
1. (Easy) Explain the purpose of Count in this code.
Count ← 0
FOR Index ← 1 TO 4
INPUT Age
IF Age >= 18 THEN
Count ← Count + 1
ENDIF
NEXT Index
OUTPUT CountShow answer
Count is a counter. It holds how many of the ages entered so far are 18 or over. It starts at 0 because none have been counted yet, increases by 1 only when Age >= 18 is true, and is output after the loop as the final number.
2. (Easy) A search sets Found ← FALSE before a loop and Found ← TRUE when the target is seen. Explain why Found is needed.
Show answer
Found is a flag that records whether the target has been seen. It starts FALSE because nothing is found yet. Once set to TRUE it stays TRUE, so after the loop the program can test it to decide whether to output “Found” or “Not found”.
3. (Easy) Requirement: a seat number must be a whole number from 1 to 40 inclusive. Give four boundary tests with expected results and reasons.
Show answer
- 1: accepted, because “from 1” includes the lower limit.
- 40: accepted, because “to 40 inclusive” includes the upper limit.
- 0: rejected, because it is just below the lower limit.
- 41: rejected, because it is just above the upper limit.
4. (Medium) Requirement: delivery is free when the order total is RM150 or more. Give three tests, with expected result and reason.
Show answer
- RM149.99: delivery charged, because it is just below the limit.
- RM150: free delivery, because “or more” includes exactly RM150. This catches a wrong
>instead of>=. - RM200: free delivery, as a normal value well inside the rule.
5. (Medium) Describe the correction. The prices 5, 6 and 7 should give 18 but the code outputs 7.
1 Total ← 0
2 FOR I ← 1 TO 3
3 INPUT Price
4 Total ← Price
5 NEXT I
6 OUTPUT TotalShow answer
Line 4 replaces Total with the latest Price instead of adding to it, so only the last price, 7, is left. Change it to Total ← Total + Price. The values then become 5, 11, 18, which is the expected output.
6. (Medium) This code should output 1, 2, 3 and then stop, but it never ends. Describe the correction.
1 Count ← 1
2 WHILE Count <= 3 DO
3 OUTPUT Count
4 ENDWHILEShow answer
Count is never changed inside the loop, so Count <= 3 stays true forever and 1 is output again and again. Add Count ← Count + 1 as the last statement inside the loop, after the OUTPUT. Then Count goes 1, 2, 3, 4 and the loop ends after outputting 1, 2, 3.
7. (Medium) Trace the code, state the output and explain it in one sentence.
X ← 3
Y ← 0
WHILE X > 0 DO
Y ← Y + X * X
X ← X - 1
ENDWHILE
OUTPUT YShow answer
| Pass | X > 0 ? | Y after | X after |
|---|---|---|---|
| Start | 0 | 3 | |
| 1 | 3 > 0 true | 9 | 2 |
| 2 | 2 > 0 true | 13 | 1 |
| 3 | 1 > 0 true | 14 | 0 |
| Test | 0 > 0 false | 14 | 0 |
Y adds the squares 9, 4 and 1. The loop stops when X becomes 0, and OUTPUT Y prints 14.
8. (Medium) Trace the code and give both outputs.
Even ← 0
Odd ← 0
FOR N ← 1 TO 6
IF N MOD 2 = 0 THEN
Even ← Even + N
ELSE
Odd ← Odd + N
ENDIF
NEXT N
OUTPUT Even
OUTPUT OddShow answer
N = 1: Odd 1. N = 2: Even 2. N = 3: Odd 4. N = 4: Even 6. N = 5: Odd 9. N = 6: Even 12. Even collects 2 + 4 + 6 and Odd collects 1 + 3 + 5. The outputs are 12 and then 9.
9. (Hard) A student says the output of question 7 is 0 “because X ends at 0”. Explain the error.
Show answer
The student has read the wrong variable. 0 is the final value of X, which ended the loop, but the statement is OUTPUT Y. The last value in the Y column is 14, so the output is 14. The student should name the variable in the OUTPUT statement first, then read its column.
10. (Hard) A long program should count values above 10. For the data 5, 10, 15 it outputs 2. Write a minimal reproducible example with expected and actual output, and describe the correction.
Show answer
Data[1] ← 5
Data[2] ← 10
Data[3] ← 15
Count ← 0
FOR I ← 1 TO 3
IF Data[I] >= 10 THEN
Count ← Count + 1
ENDIF
NEXT I
OUTPUT CountExpected: 1 (only 15 is above 10). Actual: 2 (10 and 15). The condition uses >=, which includes 10, but the requirement says “above 10”. Change it to IF Data[I] > 10 THEN. Data on the boundary, 10, is what exposes the fault.
11. (Hard) A test plan for “a score from 0 to 100 inclusive” uses 0, 50, 100 and 101. State what is missing and why it matters.
Show answer
There is no test just below the lower limit, −1. Without it, a rule that wrongly accepts negative scores would pass the plan. A wrong-type value such as “ten” would also be useful, because the rule says the score is a number. Add −1 (rejected, below the lower limit) and “ten” (rejected, not a number).
If you got these wrong
- Questions 1 and 2: your answer repeated the name or the code, or left out why the starting value matters. Review explain the purpose of a variable.
- Questions 3, 4 and 11: you tested values without a reason, or missed a boundary. Review justify a test case with a requirement.
- Questions 5 and 6: the fault or the fix was described loosely. Review describe a correction rather than merely show code.
- Questions 7, 8 and 9: the table was right but the output was misread. Review connect a trace with final output.
- Question 10: the example was too large, too small, or had no expected output. Review prepare a minimal reproducible learning example.
Use the mistake log and retest queue to note the type of error and retest it a few days later. The module overview, problem-solving explanations, shows the full route.
If you would like someone to read your written answers alongside the mark points, our teachers can do that in online one-to-one Computer Science tuition.