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From Gilles Sadowski <gil...@harfang.homelinux.org>
Subject Re: svn commit: r1348721 - in /commons/proper/math/trunk/src: main/java/org/apache/commons/math3/linear/OpenMapRealVector.java test/java/org/apache/commons/math3/linear/SparseRealVectorTest.java
Date Mon, 11 Jun 2012 16:04:48 GMT
Hello Sébastien.

> >> +        /*
> >> +         * MATH-803: it is not sufficient to loop through non zero entries
of
> >> +         * this only. Indeed, if this[i] = 0d and v[i] = 0d, then
> >> +         * this[i] / v[i] = NaN, and not 0d.
> >> +         */
> >> +        final int n = getDimension();
> >> +        for (int i = 0; i < n; i++) {
> >> +            res.setEntry(i, this.getEntry(i) / v.getEntry(i));
> >>          }
> >>          return res;
> >>      }
> >
> > I think that this renders the class potentially useless, if we assume that
> > it is used when "large" vectors are needed.
> > Indeed, after this operation, all the entries will be stored; thus
> > cancelling the memory efficiency of the class, and probably making the
> > returned value slower than an "ArrayRealVector" instance for subsequent
> > operations.
> >
> I'm not sure that all entries would be stored (except if setEntry does
> not distinguish between zero values and non-zero values).

The problem is when the common values are not the default one, like ...

> 
> > For every method that a "RealVector" argument, there should be a specialized
> > implementation that take an "OpenMapRealVector".
> >
> > Also, couldn't we solve some of these problems if the value of the default
> > entry was stored and mutable? E.g. if the default for "v" and "w" is zero,
> > then the default for "v / w" will be "NaN".

... in this example.

> >
> >> @@ -359,6 +363,25 @@ public class OpenMapRealVector extends S
> >>              iter.advance();
> >>              res.setEntry(iter.key(), iter.value() * v.getEntry(iter.key()));
> >>          }
> >> +        /*
> >> +         * MATH-803: the above loop assumes that 0d * x  = 0d for any
double x,
> >> +         * which allows to consider only the non-zero entries of this.
However,
> >> +         * this fails if this[i] == 0d and (v[i] = NaN or v[i] = Infinity).
> >> +         *
> >> +         * These special cases are handled below.
> >> +         */
> >> +        if (v.isNaN() || v.isInfinite()) {
> >> +            final int n = getDimension();
> >> +            for (int i = 0; i < n; i++) {
> >> +                final double y = v.getEntry(i);
> >> +                if (Double.isNaN(y)) {
> >> +                    res.setEntry(i, Double.NaN);
> >> +                } else if (Double.isInfinite(y)) {
> >> +                    final double x = this.getEntry(i);
> >> +                    res.setEntry(i, x * y);
> >> +                }
> >> +            }
> >> +        }
> >
> > This probably can only be a temporary solution: 3 additional loops over all
> > the elements...
> >
> If we cache isNaN and isInfinite, this is "only" one additional loop ;).
> I don't like this solution either, but the bugs I have identified are
> very real, except if we accept that exceptional cases are not handled
> correctly by sparse vectors. I'm personally adverse to this option.
> If you have a better idea, please feel free to change that. By the
> way, would you like me to revert this commit ?

No. But maybe we should already open a ticket signalling that a potential
problem has been identified.

> 
> I just would like to mention that sparse vectors allow a gain in
> memory. Speed is, in my view, a bonus, but you probably think the
> opposite...

No, that's fine; less so if you lose both. ;-)


Best regards,
Gilles

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