R help - Dec 2001 - Extracting/setting elements from/in a matrix/array

Dear all,


I had to extracts/set elements from/in a matrix. Let say I have two
vectors dim1 and dim2 of indices in the respective two dimensions of a
matrix: I want to extract all the corresponding elements. I the case of a
nxn matrix, dim1 <- 1:n and dim2 <- 1:n would extract the diagonal.

I know one way would be to use the functions 'row' and 'col',
but the
matrixes I have are can be rather large. This is probably a detail but
I was looking for something that would avoid be bit more memory
friendly... I have made functions for that purpose but I suspect somebody
already did something (which would be in R, but I could not find it...).
For the sake of avoiding duplication of similar things (hence making code 
more readable), I would be happy to replace it in my code.

Does anybody knows if such things already exists ?


Laurent



Laurent Gautier			CBS, Building 208, DTU
PhD. Student			D-2800 Lyngby,Denmark	
tel: +45 45 25 24 85		http://www.cbs.dtu.dk/laurent

-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at
stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._

>>>>> "Laurent" == Laurent Gautier <laurent at
genome.cbs.dtu.dk> writes:
    Laurent> Dear all,


    Laurent> I had to extracts/set elements from/in a
    Laurent> matrix. Let say I have two vectors dim1 and dim2 of
    Laurent> indices in the respective two dimensions of a
    Laurent> matrix: I want to extract all the corresponding
    Laurent> elements. I the case of a nxn matrix, dim1 <- 1:n
    Laurent> and dim2 <- 1:n would extract the diagonal.

The solution to your problem has been part of the   S language
for ever :

    mat[ cbind(dim1, dim2) ]

Note that this is mentioned in help("[") and MASS and ...

    Laurent> I know one way would be to use the functions 'row'
    Laurent> and 'col', but the matrixes I have are can be
    Laurent> rather large. This is probably a detail but I was
    Laurent> looking for something that would avoid be bit more
    Laurent> memory friendly... I have made functions for that
    Laurent> purpose but I suspect somebody already did
    Laurent> something (which would be in R, but I could not
    Laurent> find it...).  For the sake of avoiding duplication
    Laurent> of similar things (hence making code more
    Laurent> readable), I would be happy to replace it in my
    Laurent> code.

    Laurent> Does anybody knows if such things already exists ?


    Laurent> Laurent Gautier CBS, Building 208, DTU PhD. Student
    Laurent> D-2800 Lyngby,Denmark tel: +45 45 25 24 85
    Laurent> http://www.cbs.dtu.dk/laurent
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at
stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._

Laurent Gautier <laurent at genome.cbs.dtu.dk> writes:
> Dear all,
> 
> 
> I had to extracts/set elements from/in a matrix. Let say I have two
> vectors dim1 and dim2 of indices in the respective two dimensions of a
> matrix: I want to extract all the corresponding elements. I the case of a
> nxn matrix, dim1 <- 1:n and dim2 <- 1:n would extract the diagonal.
> 
> I know one way would be to use the functions 'row' and
'col', but the
> matrixes I have are can be rather large. This is probably a detail but
> I was looking for something that would avoid be bit more memory
> friendly... I have made functions for that purpose but I suspect somebody
> already did something (which would be in R, but I could not find it...).
> For the sake of avoiding duplication of similar things (hence making code 
> more readable), I would be happy to replace it in my code.
> 
> Does anybody knows if such things already exists ?
It does:  A[cbind(dim1,dim2)]

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at
stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._