Test Valid and Invalid Inputs Separately

Supplied materials

• Paper and pencil; an adult may read aloud. No device, account or purchase is needed.

Learn the idea

A function contract states which inputs are allowed and what outputs should mean. Tests should cover both accepted and rejected groups. Here count must be an integer from 0 through 8, excluding Boolean values; return invalid for anything else. For valid counts below 4 return small; at least 4 return large.

Pseudocode: if count is not an allowed integer, return invalid; if count < 4, return small; otherwise return large. The lower input boundary 0, classification boundary 4 and upper input boundary 8 are separate. A value can be within the input contract but on either side of the classification threshold.

1. Write test rows for −1, 0, 3, 4, 8, 9 and 2.5.
2. Mark each row valid or invalid before classification.
3. Add a text input "4" and a Boolean true to confirm they are rejected rather than coerced.
4. Record expected returns and explain which boundary or input group each test covers.

Check your reasoning

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Take it further

Change the allowed upper limit to 6, leaving the classification threshold 4. Expected tests: 6 → large; 7 → invalid. Previous input 8 now becomes invalid.

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