R help - May 2013 - Extracting elements from a matrix using a vector containing indices

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?
-- 
mcoletti@gmail.com

	[[alternative HTML version deleted]]

On 07-05-2013, at 06:39, Mark Coletti <mcoletti at gmail.com> 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.
> 
It's not clear what you want.
The vector C contains row numbers or column numbers?
> 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.
So you want to extract a single element from each column?
Your matrix is Mat and the vector is called vec

Mat[vec,]

should do it.

If this is not what you want you will have to be more specific.

Berend
>  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?
> -- 
> mcoletti at gmail.com
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
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> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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