Values, expressions, and arithmetic — Boundary detective

PART I

Warm-up

Reconnect with a useful idea before starting values, expressions, and arithmetic. In one or two sentences, describe the input, output or host boundary from the previous exercise that this lesson builds on. If you are arriving directly, skim the worked example first; there is no locked prerequisite.

A rough note is enough. You can revise it after the example.

PART II

Instruction & guided example

Use zero to audit a formula

Zero can remove a term and expose a formula’s structure. In earned + count * rewardEach, setting count to zero should leave earned. Setting earned to zero should leave the bonus product. These are separate expectations, not an instruction to test only empty inputs.

For score(9,0,4), count * rewardEach becomes 0 * 4 = 0, then 9 + 0 = 9. For score(0,3,4), the product is 12 and total 12. A mistaken (earned + count) * rewardEach gives 36 for the first case, revealing that earned was multiplied too.

The all-zero case returns zero under many wrong formulas and is weak on its own. Use a case that leaves one nonzero meaningful quantity visible. Large random inputs often obscure the same defect that a carefully chosen zero reveals immediately.

Worked reasoning and supplied artifact

fnc score(earned: i32, count: i32, rewardEach: i32) -> i32 {
    return earned + count * rewardEach;
}

Illustrative example from the proposed WillowScript design. The language and WUIX/WMATH/WDB/WAUDIO/WTMP services are not implemented in the supplied source. This site has no interpreter, VM or native host and does not execute or certify the snippet.

Try the guided change

Trace (9,0,4) and (0,3,4), and identify the exact subexpression that becomes zero.

• Write the supplied inputs and initial state before following the example.
• Reconstruct each intermediate calculation or host decision, then compare with the worked artifact.
• Explain why the contrasting alternative fails before attempting the changed inputs in Part IV.

PART III

Take a breather

Step back after the guided example. Look away from the screen, stretch, or reread your prediction. This is an optional learning pause, not a timer or an assessment gate.

When you return, try to name the lesson’s key idea without copying its wording. If that feels difficult, return to Part II and trace one line rather than restarting the entire lesson.

PART IV

Independent practice

Project context: Snack-shop calculator. Build a small, understandable change around values, expressions, and arithmetic.

For a delivery score with starting points plus parcels times points-per-parcel, design three zero cases. State which one is least informative and add a nonzero case that distinguishes the correct formula from multiplying the starting points.

What to produce

A case matrix with expected values and the defect each row can or cannot expose.

• State the intended behavior in a sentence before editing.
• Use only source-backed syntax and the host services whose contract you have been given.
• Record a normal case and a boundary or unavailable-resource case.
• Label hand predictions separately from anything actually measured in a supplied toolchain.

Syntax colors are a reading aid, not a compiler. The line you are editing stays uncolored until you move to a different line; capitalization is never changed. Drafts stay on this browser and code is not executed.

fnc add(a: i32, b: i32) -> i32 {
    return a + b;
}
fnc main() -> i32 {
    return add(2, 3);
}

PART V

Self-check

Use this check to notice what to revisit. It does not award a private course grade, block another page or claim to verify compiled code. Every answer has a learning explanation.

You may keep exploring even if you are unsure. Use the hint, compare the trace, and revise your explanation.

PART VI

Recap & small task

Bring the idea back to the mod feature you are building. A useful recap connects the mechanism to a decision you can make in your own code.

• Use zero to audit a formula
• Zero can remove a term and expose a formula’s structure. In earned + count * rewardEach, setting count to zero should leave earned. Setting earned to zero should leave the bonus product. These are separate expectations, not an instruction to test only empty inputs.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Explain why an all-zero case can pass a broken formula.

This marks your personal practice record only. It is not a certification or an official workspace assessment.