R help - Oct 2000 - Extracting vectors from matrices

There is probably a simple and obvious answer to this, so sorry for
wasting your time...

I have a matrix of predictions from an proportional odds model (using
the polr function in MASS), so the columns are the probabilities of the
responses, and the rows are the data points.  I have another column with
the observed responses, and I want to extract the probabilities for the
observed responses.

As a toy example, if I have 
x <- matrix(c(1,2,3,4,5,6),2,3)
y <- c(1,3)
and I want to extract the numbers in x[1,1] and x[2,3] (the columns
being indexed from y), what do I do?

Bob

-- 
Bob O'Hara
Metapopulation Research Group
Division of Population Biology
Department of Ecology and Systematics
PO Box 17 (Arkadiankatu 7)
FIN-00014 University of Helsinki
Finland

tel: +358 9 191 7382  fax: +358 9 191 7301 
email: bob.ohara at helsinki.fi
To induce catatonia, visit:
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I have yet to see any problem, however complicated, which, when you
looked at it in the right way, did not become still more complicated.  -
Poul Anderson
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On Fri, 27 Oct 2000, Anon. (possibly Bob O'Hara with a broken
mailer) wrote:
> There is probably a simple and obvious answer to this, so sorry for
> wasting your time...
> 
> I have a matrix of predictions from an proportional odds model (using
> the polr function in MASS), so the columns are the probabilities of the
> responses, and the rows are the data points.  I have another column with
> the observed responses, and I want to extract the probabilities for the
> observed responses.
> 
> As a toy example, if I have 
> x <- matrix(c(1,2,3,4,5,6),2,3)
> y <- c(1,3)
> and I want to extract the numbers in x[1,1] and x[2,3] (the columns
> being indexed from y), what do I do?
I don't see a unique pattern to this, but is

 x[cbind(seq(along=y), y)]

what you had in mind?  The key is definitely matrix indexing.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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