[ https://issues.apache.org/jira/browse/MATH-836?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Baste Nesse Buanes updated MATH-836:
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Attachment: value-maxDenominator_pairs_that_fails
pairs of doubles and ints that makes the constructor Fraction(double, int) return faulty Fractions.
> Fraction(double, int) constructor strange behaviour
> ---------------------------------------------------
>
> Key: MATH-836
> URL: https://issues.apache.org/jira/browse/MATH-836
> Project: Commons Math
> Issue Type: Bug
> Affects Versions: 3.0
> Reporter: Baste Nesse Buanes
> Priority: Minor
> Labels: Fraction
> Attachments: FractionTestByAxiom.java, value-maxDenominator_pairs_that_fails
>
>
> The Fraction constructor Fraction(double, int) takes a double value and a int maximal denominator, and approximates a fraction. When the double value is a large, negative number with many digits in the fractional part, and the maximal denominator is a big, positive integer (in the 100'000s), two distinct bugs can manifest:
> 1: the constructor returns a positive Fraction. Calling Fraction(-33655.1677817278, 371880) returns the fraction 410517235/243036, which both has the wrong sign, and is far away from the absolute value of the given value
> 2: the constructor does not manage to reduce the Fraction properly. Calling Fraction(-43979.60679604749, 366081) returns the fraction -1651878166/256677, which should have* been reduced to -24654898/3831.
> I have, as of yet, not found a solution. The constructor looks like this:
> public Fraction(double value, int maxDenominator)
> throws FractionConversionException
> {
> this(value, 0, maxDenominator, 100);
> }
> Increasing the 100 value (max iterations) does not fix the problem for all cases. Changing the 0-value (the epsilon, maximum allowed error) to something small does not work either, as this breaks the tests in FractionTest.
> The problem is not neccissarily that the algorithm is unable to approximate a fraction correctly. A solution where a FractionConversionException had been thrown in each of these examples would probably be the best solution if an improvement on the approximation algorithm turns out to be hard to find.
> This bug has been found when trying to explore the idea of axiom-based testing (http://bldl.ii.uib.no/testing.html). Attached is a java test class FractionTestByAxiom (junit, goes into org.apache.commons.math3.fraction) which shows these bugs through a simplified approach to this kind of testing, and a text file describing some of the value/maxDenominator combinations which causes one of these failures.
> * It is never specified in the documentation that the Fraction class guarantees that completely reduced rational numbers are constructed, but a comment inside the equals method claims that "since fractions are always in lowest terms, numerators and can be compared directly for equality", so it seems like this is the intention.
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