[Rd] [External] Re: svd() results should have a class

[Duncan Murdoch]

On 23/06/2022 8:07 p.m., Lenth, Russell V wrote:
> Bob,
> 
> I'm not talking about using svd as a generic method. I am talking about a method FOR svd results, e.g. an S3 method like foo.svd(), for which there already exist other methods, say foo.default and foo.qr. Currently if I wanted to do
> 
>      svdobj <- svd(x)
>      foo(svdobj)
> 
> it would not dispatch correctly because there is no svd class. Instead, it would be handled by foo.list if it exists, and it is certainly not clear that foo.list would do the right thing.

I think this would be more convincing if you gave a real example.  You 
can see the existing methods for the "qr" and "eigen" classes using

   methods(class = "qr")
   methods(class = "eigen")

When I do it, I think I'm only seeing methods from base packages, and 
they are:

   kappa for qr
   solve for qr
   print for eigen

I think the default print method works fine for svd() results. 
kappa.svd would probably make sense, but would need some thought: 
calculating kappa on a matrix and using the help page for kappa to 
naively calculate it from the svd() of that matrix give different results:

   > set.seed(123)
   > X <- matrix(rnorm(25), 5,5)
   > kappa(X)
   [1] 90.71283
   > s <- svd(X)
   > s$d[1]/s$d[length(s$d)]
   [1] 62.68048

Duncan Murdoch

> Russ
> 
> Sent from my iPad
> 
> On Jun 23, 2022, at 6:53 PM, Robert Harlow <rharlow86 using gmail.com> wrote:
> 
> 
> Don't have a view on whether it makes sense in base R or not, but WRE section 7.1 may be helpful to you: https://cran.r-project.org/doc/manuals/R-exts.html#Adding-new-generics.
> 
> It's not uncommon for packages to want to make base methods generic and the above link provides advice on how to do so.
> 
> Bob
> 
> On Thu, Jun 23, 2022 at 12:07 PM Lenth, Russell V <russell-lenth using uiowa.edu<mailto:russell-lenth using uiowa.edu>> wrote:
> Dear R-Devel,
> 
> I noticed that if we run base::svd(x), we obtain an object of class "list". Shouldn't there be an "svd" class, in case someone (e.g., me) wants to write methods for singular value decompositions? Note that other matrix-decomposition routines like qr() and eigen() each return objects having those names.
> 
> Thanks
> 
> Russ Lenth
> Russell-lenth using uiowa.edu<mailto:Russell-lenth using uiowa.edu>
> 
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