Choose loops and validate input
Match the loop to the requirement
Use FOR for a known number of accepted items, WHILE when zero iterations are possible, and REPEAT when reading at least once is required. A sentinel marks the end of input; it is not ordinary data.
Separate attempts from accepted items
To read three valid scores: the outer loop counts accepted scores; the inner REPEAT handles attempts for one score. Add to Total only after validation succeeds.
DECLARE Score, Total, I : INTEGER
Total ← 0
FOR I ← 1 TO 3
Score ← I * 2
Total ← Total + Score
NEXT I
OUTPUT Total
This fixed-data version gives 12. Replacing the assignment with a validation loop should not change where the total is updated.
Test the decisions
For the inclusive interval 0 to 20 use -1, 0, 20 and 21 as boundary tests. An invalid attempt must neither increase the accepted-item count nor contribute to the total. Explain why UNTIL uses the valid condition, rather than the invalid condition.
A system must read at least one value and repeat until it is valid. Which loop most directly expresses this: FOR, WHILE or REPEAT?
Read integers until one is 1 to 9 inclusive. Output only the accepted value.
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Read N (1 to 5). For EACH score, read repeatedly until it is 0 to 20 inclusive. Output the total of the N accepted scores. Do not count invalid attempts.
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Explain your choice of data structure and loop. Identify one boundary case and predict its result. For a subroutine, explain what is returned or changed in the caller.
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Review your explanation against these criteria
- My types and data structure match the data and the question.
- I justified the loop and checked its first and last iterations.
- Initial values and rejected operations preserve the intended state.
- My test includes input, purpose and expected result.
- I distinguished returning a value from printing and changing caller state.