# [Rd] non user-friendly error for chol2inv functions

(Ted Harding) Ted.Harding at manchester.ac.uk
Fri Aug 29 15:21:05 CEST 2008

```On 29-Aug-08 13:00:01, Martin Maechler wrote:
>>>>>> "cd" == christophe dutang <dutangc at gmail.com>
>>>>>>     on Fri, 29 Aug 2008 14:28:42 +0200 writes:
>
> cd> Yes, I do not cast the first argument as a matrix with
> cd> as.matrix function.
> cd> Maybe we could detail the error message if the first
> cd> argument
> cd> is a numeric?
>
> cd> error(_("'a' is a numeric and must be coerced to a numeric
> cd> matrix"));
>
> Merci, Christophe.   Yes, we *could* do that.
> Alternatively, I think I will just make it work in that case,
> since I see that
>       qr(), chol(), svd(), solve()  all
> treat a numeric (of length 1) as a  1 x 1 matrix automatically.

I was about to draw attention to this "inconsistency"!
While one is about it, might it not be useful also to do
the converse: Treat a 1x1 matrix as a scalar in appropriate
contexts?

E.g.

a <- 4
A <- matrix(4,1,1)
B <- matrix(c(1,2,3,4),2,2)
a*B
#      [,1] [,2]
# [1,]    4   12
# [2,]    8   16

a+B
#      [,1] [,2]
# [1,]    5    7
# [2,]    6    8

A*B
# Error in A * B : non-conformable arrays
A+B
# Error in A + B : non-conformable arrays

Ted.

>
> De rien!
> Martin
>
>     cd> 2008/8/29 Martin Maechler <maechler at stat.math.ethz.ch>
>
>     >> >>>>> "cd" == christophe dutang <dutangc at gmail.com>
>     >> >>>>>     on Fri, 29 Aug 2008 12:44:18 +0200 writes:
>     >>
>     cd> Hi,
>     cd> In function chol2inv with the option LINPACK set to false
> (default),
>     >> it
>     cd> raises an error when the matrix is 1x1 matrix (i.e. just a
> real)
>     >> saying
>     >>
>     cd> 'a' must be a numeric matrix
>     >>
>     >> It is very helpful, but you have to read and understand it.
>     >> I'm pretty sure you did not provide a  1 x 1 matrix.
>     >>
>     >> Here's an example showing how things works :
>     >>
>     >> > m <- matrix(4,1,1)
>     >> > cm <- chol(m)
>     >> > cm
>     >> [,1]
>     >> [1,]    2
>     >> > chol2inv(cm)
>     >> [,1]
>     >> [1,] 0.25
>     >> >
>     >>
>     >> Martin Maechler, ETH Zurich
>     >>
>     >>
>     cd> This error is raised by the underlying C function
> (modLa_chol2inv in
>     cd> function Lapack.c). Everything is normal, but I wonder if we
> could
>     >> have
>     cd> another behavior when we pass a 1x1 matrix. I spent time this
>     >> morning
>     cd> finding where was the error, and it was this "problem".
>     >>
>     >>
>     cd> Christophe
>     >>
>     cd> [[alternative HTML version deleted]]
>     >>
>     cd> ______________________________________________
>     cd> R-devel at r-project.org mailing list
>     cd> https://stat.ethz.ch/mailman/listinfo/r-devel
>     >>
>
>     cd> [[alternative HTML version deleted]]
>
>     cd> ______________________________________________
>     cd> R-devel at r-project.org mailing list
>     cd> https://stat.ethz.ch/mailman/listinfo/r-devel
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
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Date: 29-Aug-08                                       Time: 14:16:03
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