Using a list as multidimensional indexer

Hello list.

Another 'puzzle' for which I don't have a clean solution.
Say I have a multidimensional object, e.g.:
Mm<-matrix(1:6, nrow=2, dimnames=list(c("a","b"), c("g","h","i")))
And on the other hand I have a list
Ind<-list("b","g")
This holds, for each dimension, an indexer for that dimension.
Now I would like to get the element pointed at by the list.
The obvious solutions don't seem to work, and I can't seem to get do.call to
call the indexer ('[') on my multidimensional object.

Any suggestions?

do.call(`[`, c(quote(Mm), Ind))

[1] 2

do.call(`[`, c(list(Mm), Ind))

[1] 2

gives

Mm["b", "g"]

[1] 2

The first refers to the object by name, the second uses it directly.

Thanks in advance,

Nick Sabbe
--
ping: nick.sabbe at ugent.be
link: http://biomath.ugent.be

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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Hm. I got somewhat further:

Ind2<-list(Mm,"b","g")
do.call("[",Ind2)
Seems to work.

However, now I need it one step beyond: in fact, my actual multidimensional
object holds one dimension more than my list holds indexes.
i.e.: I want the equivalent of Mm["a",].

I tried some variants of
Ind3<-list(Mm,"b",NULL)
do.call("[", Ind3)
But all of these return integer(0).

So the actual new question is: how do I pass a 'missing' argument through a
do.call?

Thanks for any pointers,


Nick Sabbe
--
ping: nick.sabbe at ugent.be
link: http://biomath.ugent.be
wink: A1.056, Coupure Links 653, 9000 Gent
ring: 09/264.59.36

-- Do Not Disapprove




-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of Nick Sabbe
Sent: donderdag 20 januari 2011 11:05
To: r-help at r-project.org
Subject: [R] Using a list as multidimensional indexer

Hello list.

Another 'puzzle' for which I don't have a clean solution.
Say I have a multidimensional object, e.g.:
Mm<-matrix(1:6, nrow=2, dimnames=list(c("a","b"), c("g","h","i")))
And on the other hand I have a list
Ind<-list("b","g")
This holds, for each dimension, an indexer for that dimension.
Now I would like to get the element pointed at by the list.
The obvious solutions don't seem to work, and I can't seem to get do.call to
call the indexer ('[') on my multidimensional object.

Any suggestions?

Thanks in advance,

Nick Sabbe
--
ping: nick.sabbe at ugent.be
link: http://biomath.ugent.be
wink: A1.056, Coupure Links 653, 9000 Gent
ring: 09/264.59.36

-- Do Not Disapprove

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