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SECTION 12 / 15
Side effects: returning a number can change a deck
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A procedure may return the right sum and still change the data arrangement. That extra change is called a side effect. To use the procedure safely, the caller must know what happens to its input.
Pass the data too
SumMarks(Gender,Field) assumes a particular deck already exists. To reuse it for another class, also pass the dataset: SumMarks(Deck,Gender,Field). State whether the procedure may consume, reorder or edit that deck.
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Watch a procedure change a deck
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All cards returned is not the same as original order
Start with top-to-bottom IDs 0,1,2. Move each top card onto the top of SeenDeck: the new top-to-bottom order is 2,1,0. Renaming SeenDeck as Deck gives every card back, but in reverse order. A second top-to-top transfer reverses it again and restores 0,1,2.
For a sum, order does not change the answer: 68+62 equals 62+68. However, leaving the original deck empty breaks a later call unless the interface explicitly says it consumes the deck. In the lab, compare “Move cards back one at a time” with “Rename the seen pile”. Both retain the marks sum 2,171, but only the former restores sequence.
Not every procedure needs a useful return value
UpdateMarks(CardNo,Field,NewValue) may be called as a statement because its purpose is to change a record. If a mark changes, keep the stored Total consistent too. Sorting also intentionally changes order; for such a procedure the change is its goal, not an unwanted surprise.
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Can a correct sum hide an unwanted side effect?
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Yes. The sum can be correct while the input deck is emptied or reordered.
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What order results from moving 0,1,2 top-to-top into a new pile?
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2,1,0, read top-to-bottom.
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Does renaming a reversed pile restore sequence?
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No. It returns the cards but preserves the reversed sequence.
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Why pass Deck as an argument?
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So the same procedure can work on different datasets rather than one hidden fixed pile.
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Is reordering always a bug?
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No. Sorting intentionally reorders data. Whether a change is acceptable depends on the contract.