Test Both Sides of a Decision
Choose cases that exercise both branches and find a reversed action.
Supplied materials
- Paper and pencil; an adult may read aloud. No device, account or purchase is needed.
Learn the idea
Testing one case can leave part of a rule untested. For if/else, choose at least one case where the condition is true and one where it is false. Write the expected output from the stated goal before tracing the draft. This separates what should happen from what the draft actually does.
Goal rule: if a card has a dot, write D; else write N. Faulty draft: if a card has a dot, write N; else write N. The supplied dot card should give D and the blank card should give N. The blank test passes, while the dot test shows the wrong true-branch action.
- Draw one dot card and one blank card.
- Write expected outputs D and N from the goal rule.
- Trace the faulty draft and record actual outputs N and N.
- Replace the true-branch N with D, then retest both cards.
See the method in action
Before repair: dot card expected D, actual N, fail; blank card expected N, actual N, pass. After repair, the dot card gives D and the blank card gives N: both tests pass.
Check your reasoning
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See the answer and explanation
The dot card should give D
Only a card with a dot reaches the incorrect true branch. A blank card exercises the already-correct else branch.
Take it further
Change the goal to triangle cards give T and all others give O. Test a triangle and a circle. Expected outputs: T and O; both branches are exercised.
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