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Grade 5 · lesson 2 of 4

Check an Input Before Using It

Reject values outside a stated whole-number input contract.

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Supplied materials

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

Learn the idea

Validation asks whether an input meets the rules for being used. It is different from classifying a valid input. Our fictional page counter accepts whole numbers from 0 through 30 inclusive. Negative numbers, fractions and numbers above 30 receive invalid. We use supplied invented values, not records about a learner.

Algorithm: if pages is not a whole number from 0 through 30, show invalid; otherwise, if pages ≥ 10, show Goal reached; else show Keep going. Check validity first so an impossible 31-page input does not reach the goal branch. In this exercise, words and blank inputs are also invalid.

  1. Write the accepted range and whole-number rule.
  2. Make rows for −1, 0, 9, 10, 30, 31 and 2.5.
  3. Validate each value before applying the threshold.
  4. Record one final message for each row; do not classify rejected values.
Worked example

See the method in action

Rows: −1 → invalid; 0 → Keep going; 9 → Keep going; 10 → Goal reached; 30 → Goal reached; 31 → invalid; 2.5 → invalid. Both ends of the allowed range are included.

Your turn

Check your reasoning

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Under this contract, what happens for pages = 30?

Feedback appears here after you check. This is practice, not a graded school assessment.

See the answer and explanation

Goal reached

30 is a whole number at the included upper end of the range and is at least 10. It is valid and gets Goal reached.

Take it further

Change the allowed range to 0 through 12, keeping the goal threshold 10. Test 12 and 13. Expected outputs: Goal reached and invalid.

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Check the subject sources

These LifeStarter passages, scenarios and exercises are authored for this Academy with AI assistance and review. External resources keep their publishers’ terms. A grade label suggests a starting level; it does not establish alignment with your school’s complete requirements.