[R] Extracting elements from a matrix using a vector containing indices
peter dalgaard
pdalgd at gmail.com
Tue May 7 10:53:48 CEST 2013
On May 7, 2013, at 06:39 , Mark Coletti wrote:
> I have a matrix of data that has a corresponding vector of indices. I
> would like to use those indices to extract specific matrix elements into a
> new vector. In other words, I have an R X C matrix with a corresponding
> vector of C elements that have numbers mapping into specific elements of
> the matrix. I'd like to generate a new vector C long that contains those
> elements.
>
> My first approach is to iteratively grind through each matrix column to
> manually extract the element corresponding to that column as referenced by
> the appropriate index, and then appending that to a vector. However, my R
> instincts are that any time one has to craft a for loop to do something in
> R that you are very likely Doing It Wrong; there's probably a very simple
> one liner in R that will do all that that for loop would do.
>
> I know there must be a simple "R way" to do this, but I remain flummoxed on
> how to do so. I've been feebly poking at the plyr package's maplyr() and
> colwise() since I have a matrix and want to extract an array from it, and
> this is inherently a column wise operation, so ... hmm.
>
> Should I just give up and use a for loop?
As Berend says, you're not specifying the problem very clearly. Are you looking for something like this (indexing with a matrix)?
> M <- matrix(round(rnorm(20,20,10)),4,5)
> M
[,1] [,2] [,3] [,4] [,5]
[1,] 29 19 18 13 0
[2,] 24 24 11 25 10
[3,] 20 9 12 11 24
[4,] 28 3 16 17 32
> Ix <- sample(1:4,5,replace=TRUE)
> Ix
[1] 3 2 2 4 3
> M[cbind(Ix,1:5)]
[1] 20 24 11 17 24
--
Peter Dalgaard, Professor
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk Priv: PDalgd at gmail.com
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