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From "Stephen Colebourne" <scolebou...@btopenworld.com>
Subject Re: [lang] Fraction.getFraction(double) uses magic numbers
Date Tue, 12 Aug 2003 23:19:23 GMT
Bear in mind that this came from a contributer. I don't know the maths of
it!

I added the 25 limit, to avoid infinite recursion. The test cases demostrate
that the method returns the correct result for many, many cases. But I'm
sure it could be improved. For 2.0 I would suggest documenting behaviour
rather than adding a new method to control the value '25'.

Stephen


----- Original Message -----
From: "Phil Steitz" <phil@steitz.com>
To: "Jakarta Commons Developers List" <commons-dev@jakarta.apache.org>
Sent: Monday, August 11, 2003 4:24 AM
Subject: Re: [lang] Fraction.getFraction(double) uses magic numbers


> Brian S O'Neill wrote:
> > I don't understand why the continued fraction implementation exists at
all.
> > Why not just get the bits from the double floating point number directly
> > rather than introduce error? The floating point number is already a
> > fraction, just encoded specially.
>
> The point as I see it is to get the best rational approximation of a
> double value with bounded denominator.  The continued fraction
> decomposition will do this better and more efficiently than just
> reducing the fraction implied by the decimal or binary representation of
> the number.  Consider, for example, the number 0.66666, to be
> represented by a fraction with denominator <=10,000.  The direct
> approach using the decimal representation would give 6667/10000, which
> is not as good as 2/3, which you would get by continued fractions.
>
> Phil
>
> >
> > ----- Original Message -----
> > From: "Phil Steitz" <phil@steitz.com>
> > To: "Jakarta Commons Developers List" <commons-dev@jakarta.apache.org>
> > Sent: Sunday, August 10, 2003 02:48 P
> > Subject: [lang] Fraction.getFraction(double) uses magic numbers
> >
> >
> >
> >>o.a.c.l.math.Fraction includes a getFraction factory method that takes a
> >>double and uses continued fractions to find a fractional approximation
> >>of the input.  The continued fraction implementation has a hard-coded
> >>maximum number of iterations (25) and maximum denominator (1000).  These
> >>should be documented (and the ArithmeticException if maximum iterations
> >>is reached before convergence).  Better (IMHO) would be to add another
> >>version that takes these as parameters, possibly even replacing the
> >>current method (I think this is new in 2.0, so there would be no problem
> >>with backward compatability).
> >>
> >>If there are no objections, I will submit a patch that clarifies current
> >>behavior and adds another method that takes maximum iterations and
> >>maximum denominator as additional parameters.
> >>
> >>I would also like to improve the implementation, but this can wait until
> >>after 2.0.
> >>
> >>Phil
> >>
> >>
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> >
> >
> >
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>
>
>
>
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