[R] Extracting elements from a matrix using a vector containing indices
David Winsemius
dwinsemius at comcast.net
Tue May 7 21:20:38 CEST 2013
On May 7, 2013, at 11:52 AM, Mark Coletti wrote:
> On Tue, May 7, 2013 at 4:53 AM, peter dalgaard <pdalgd at gmail.com> 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.
So read ?Extract more carefully.
It's a very important help page and failing to read it (ideally least 5 times) results in many, many questions on this list.
--
David Winsemius
Alameda, CA, USA
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