[R] proportional matrix rows

[Robin Hankin]

Hi Peter, Gabor

On Feb 7, 2005, at 04:42 pm, Peter Dalgaard wrote:

> Gabor Grothendieck <ggrothendieck at myway.com> writes:
>
>> Robin Hankin <r.hankin <at> soc.soton.ac.uk> writes:

>> : I want a diagnostic that detects whether a row is a multiple of
>> : the first row or not.

> ..and in that case it might be easier just to calculate the dot
> product with the orthogonal of mat[1,]:
>
>> z <- jj[1,]
>> jj %*% c(z[1], -z[2])
>       [,1]
>  [1,]    0
>  [2,]    0
>  [3,]    1
>  [4,]    1
>  [5,]    1
>  [6,]    1
>  [7,]    1
>  [8,]    0
>  [9,]    1
> [10,]    1
>
> (Notice that the value "1" just comes from Robin being singularly bad
> at selecting "arbitrary" rows that are not proportional to the first
> one!)
>

Heh.  I spent some considerable time pondering this and realized that 
if one
views jj as a bunch of two-by two matrices  jj[c(a,b),]  then in my 
application,
many of these matrices are either singular or unimodular (that is, a 
matrix with a
determinant of one).

Your insight above is is VERY VERY helpful

Thank you.


--
Robin Hankin
Uncertainty Analyst
Southampton Oceanography Centre
European Way, Southampton SO14 3ZH, UK
  tel  023-8059-7743