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From Ted Dunning <ted.dunn...@gmail.com>
Subject Re: [math] Equivalent of Blas DAXPY
Date Thu, 07 Jul 2011 17:40:27 GMT
The Colt package used a functional style to implement what you suggest.
 This approach has been inherited by Mahout.

The basic idea is that there is an assign method that takes other arguments
and a function for combining the arguments.  The function allows great
amounts of flexibility and promotes compositional style.  A small amount of
special case code for common operations and the JVM's very impressive
ability to in-line code result in pretty fast operations.

The problem with this kind of approach is that users have a very hard time
understanding how to write code (where is the plus method, they say.  where
is the element-wise max of two vectors?)

On Thu, Jul 7, 2011 at 7:14 AM, Gilles Sadowski <
gilles@harfang.homelinux.org> wrote:

> Hello.
>
> > in Blas, the method DAXPY can lead to very compact code. As far as I
> know, it
> > has no equivalent in RealVector. There are a few method called
> xxxxToSelf,
> > which modify the calling vector, instead of returning a new one, but
> these are
> > restricted to scalars operating on vectors.
> > In other words, there are no in-place vector-vector operations.
> > Would you think it useful to add at least one method, say
> > combineToSelf(double s, double t, RealVector v)
> > which would replace the components of this by the components of (s * this
> + t
> > * v). This would be a slight generalization of DAXPY.
>
> You should open an issue on JIRA to keep track of this feature request.
> [Not sure about the "combineToSelf" name: "combine" is used in
> "FunctionUtils" but not exactly with the same meaning...]
>
> > It would be fairly easy to implement for ArrayRealVector, not sure about
> > OpenMapRealVector.
>
> A default implementation would be implemented in "AbstractRealVector",
> (where a "SparseEntryIterator" is defined and the method "mapToSelf" is an
> example of how it is used).
>
>
> Thanks,
> Gilles
>
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