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A calculator can show a convincing answer that is wrong because it parsed a number under the wrong locale, detached a value from its unit, or kept displaying a result from an earlier state. Preventing these failures means making input rules explicit, keeping quantities and units together, and ensuring every visible result reflects the current calculation.
1. Input parsing: define what the number means
A string such as 1,234 does not have a universal numeric meaning. Depending on locale and input policy, the comma may mark thousands or a decimal fraction. A parser that guesses can turn a plausible entry into a different value without making the mistake obvious.
Platform APIs expose locale and strictness as choices. Apple’s Decimal.ParseStrategy builds parsing from a format style and locale; its documented en_US currency example accepts a grouped value such as $1,000.22. Android ICU’s DecimalFormat distinguishes lenient parsing intended for user input from strict validation, including different treatment of grouping separators. Depending on the API, a failed parse may be represented by a null result or an exception. These are different policies, not interchangeable guarantees.
Choose an input contract
- Decide whether input follows the device locale, a user-selected locale, or fixed notation. Make the choice visible when a string could be ambiguous.
- Specify accepted decimal and grouping separators, whitespace, signs, exponent notation, and intermediate editing states such as a trailing decimal separator.
- Reject ambiguous or malformed complete inputs, or resolve them through an explicit rule. Do not silently accept a valid prefix and reinterpret the rest.
- Keep parsing separate from formatting. Test parsing and display independently with representative locales and invalid input.
Locale-sensitive output is not an input specification: ECMA-402 defines formatting options such as grouping, rounding, and notation, but does not define a calculator’s input contract. A calculator should therefore specify how a string becomes a number rather than assuming its output formatter determines that behavior.
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2. Unit drift: keep the quantity attached to its unit
Unit drift occurs when a numeric value is converted but its label, selected unit, or subsequent operation is not updated consistently. A number alone is not enough to represent a measurement: the quantity and its unit must stay associated through calculation.
Apple’s Measurement.FormatStyle displays a measurement using a unit preferred by the active locale by default; its default locale can autoupdate with the current locale. Thus the same underlying quantity may be presented with a different unit or localized formatting without the quantity itself changing. Microsoft’s Calculator project lists conversion between many units as a feature. Together, these examples illustrate why calculation semantics should be separate from localized presentation.
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Make conversion boundaries explicit
- Represent a measurement as a typed quantity or an explicit value-plus-unit pair.
- Convert at a defined point, with both source and target units known; format only after the target unit is settled.
- Keep the displayed unit label tied to the value it describes, including after edits, mode changes, and locale changes.
- Decide whether rounding occurs during conversion or only when formatting the result.
Displayed digits are not proof of computational or physical accuracy. The GNU Units manual warns that excessive displayed precision can expose rounding artifacts and that displayed digits may not be meaningful for the intended calculation. Preserve useful internal precision where appropriate, and communicate the precision of the displayed result honestly.
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A stale-result defect is a synchronization failure: the answer on screen was derived from an earlier expression, operand, unit, or mode. The Microsoft Calculator project’s architecture documentation describes a calculation engine that maintains state. Its manual testing guide covers modes and UI behavior, including language and regional number-format changes. These documents establish stateful architecture and test coverage, not a particular stale-result incident.
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Test the transitions that can invalidate an answer
For each transition below, verify that the displayed value and unit label either update from current state or are clearly marked pending or invalid. Check history and memory presentation too if the interface shows them.
- Enter an expression, then edit an operand.
- Clear the entry or backspace through it.
- Change the operation.
- Change the source or target unit.
- Switch calculator mode.
- Change locale or number-format settings.
This is a practical test matrix derived from the stateful design; it is not a claim that a specific calculator has been tested or that a documented defect occurred.
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Review the whole calculation path
When investigating a plausible but incorrect answer, check the boundaries between input, calculation, and display rather than looking only at the arithmetic operation.
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| Area | What to define or verify |
|---|---|
| Input policy | Locale, strictness, grouping and decimal separators, malformed input, and partial edits. Apple’s parsing strategy and Android ICU document different available policies. |
| Numeric behavior | Number representation, rounding boundary, and whether displayed precision is meaningful for the calculation. GNU Units discusses precision artifacts; ECMA-402 covers formatting behavior. |
| Unit semantics | Whether each quantity remains attached to its unit, and whether locale changes presentation without changing the underlying quantity. |
| State freshness | Whether edits and setting changes refresh or invalidate the result and its unit label. Treat this as a test requirement inferred from stateful calculator architecture. |
The Microsoft Calculator project README describes “Infinite precision for basic arithmetic operations (addition, subtraction, multiplication, division) so that calculations never lose precision.” That is a stated feature of that project for those basic operations; it should not be read as a guarantee about unit conversions, display rounding, or other calculators.
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