On Tue, May 7, 2013 at 4:53 AM, peter dalgaard <[email protected]> wrote:
>
> 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.
>
> 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
I apologize for not being clearer with my question. Regardless, you were
able to suss out what I wanted -- using a vector of y coordinates to pull
individual elements out by column from a matrix. Your use of cbind() to
effect this works.
And using cbind() to extract matrix elements in that way is very
non-intuitive. There is no way I would have thought of that on my own.
Thanks!
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