# [R] extract a subset of non-contiguous elements of a matrix

PIKAL Petr petr.pikal at precheza.cz
Fri Jun 20 09:13:26 CEST 2014

```Hi

You can pad some NA to the result before transforming it to required matrix but as Frede pointed out, what do you want to do with such result?

tm.1=rbind(c(1,-3,2,-4), c(1,-3,2,-4),c(1,-3,2,-4))
xx<-which(tm.1 > 0, arr.ind=TRUE)
res<-apply(xx, 1, paste0, collapse=",")
dim(res)<-c(3,2)
> res
[,1]  [,2]
[1,] "1,1" "1,3"
[2,] "2,1" "2,3"
[3,] "3,1" "3,3"

tm.1 <- matrix(c(11,22,33,-4), ncol=2)
xx<-which(tm.1 > 0, arr.ind=TRUE)
res<-apply(xx, 1, paste0, collapse=",")
res<-c(res, NA)
dim(res)<-c(2,2)
res
[,1]  [,2]
[1,] "1,1" "1,2"
[2,] "2,1" NA

Regards
Petr

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
> project.org] On Behalf Of carol white
> Sent: Thursday, June 19, 2014 3:46 PM
> To: Frede Aakmann Tøgersen; Bart Kastermans
> Cc: r-help at r-project.org
> Subject: Re: [R] extract a subset of non-contiguous elements of a
> matrix
>
> I realize that that the problem arises if there is a different number
> of negative numbers in the rows and columns of the original matrix. In
> this case, the resulting matrix won't have the same number of rows for
> all columns. The problem for ex doesn't arise for my example but for
> Bart's example
>
> > [1,]  1  1
> > [2,]  2  1
> > [3,]  1  2
>
> how to combine these elemnts? If the 2nd col contains the number of the
> resulting matrix, then, the number of rows are different and the matrix
> can't be completed.
>
>
>
> On Thursday, June 19, 2014 3:39 PM, Frede Aakmann Tøgersen
> <frtog at vestas.com> wrote:
>
>
>
>
> As Peter and Bart I really have a problem understanding you.
>
> Perhaps if you tell us what your desired result is going to used for we
> can be more helpful. You can do that using your latest example.
>
> In that example you want a matrix of sets of row and column indices.
> This will probably have to be a matrix of characters. There you go from
> a 3×4 matrix to a 3×2 matrix.  What do you want in case of Barts 2×2
> matrix? A 3×1 or 1×3 matrix? And in a more general case?
>
> Best regards
>
> Frede
>
>
> Sendt fra Samsung mobil
>
> -------- Oprindelig meddelelse --------
> Fra: carol white
> Dato:19/06/2014 15.18 (GMT+01:00)
> Til: Bart Kastermans
> Cc: r-help at r-project.org
> Emne: Re: [R] extract a subset of non-contiguous elements of a matrix
>
>  tm.1=rbind(c(1,-3,2,-4), c(1,-3,2,-4),c(1,-3,2,-4))
>
> > which(tm.1 > 0, arr.ind=TRUE)
>      row col
> [1,]   1   1
> [2,]   2   1
> [3,]   3   1
> [4,]   1   3
> [5,]   2   3
> [6,]   3   3
>
> so the answer should have the elements of tm.1 with the following
> indexes
>
> 1,1 1,3
> 2,1 2,3
> 3,1 3,3
>
>
>
> On Thursday, June 19, 2014 3:08 PM, Bart Kastermans
> <kasterma at kasterma.net> wrote:
>
>
>
> If you give an example of input and desired output I can think about
> this.  But at this point I do not understand what you want.  In the
> example I gave the positive elements do not form a submatrix in any way
> I can think of.
>
>
> On 19 Jun 2014, at 15:04, carol white <wht_crl at yahoo.com> wrote:
>
> > well it gives a vector which is useless as I want a matrix.
> >
> >
> > On Thursday, June 19, 2014 2:40 PM, Bart Kastermans
> <kasterma at kasterma.net> wrote:
> >
> >
> > > tm.1 <- matrix(c(11,22,33,-4), ncol=2)
> > > which(tm.1 > 0, arr.ind=TRUE)
> >   row col
> > [1,]  1  1
> > [2,]  2  1
> > [3,]  1  2
> > > tm.1[which(tm.1 > 0, arr.ind=TRUE)]
> > [1] 11 22 33
> >
> > This last command does what you ask I think.
> >
> > On 19 Jun 2014, at 14:12, carol white <wht_crl at yahoo.com> wrote:
> >
> > > The extracted values don't form a matrix and that's the question
> how
> > > to extract because which returns the indexes? that is, from
> > > 1,1
> > > 2,1
> > > 1,2
> > >
> > > how to retrieve values?
> > >
> > > Or if at the position 2,1, there is a negative value, how to
> > > retrieve
> > > 1,1
> > > 1,2
> > >
> > >
> > > Carol
> > >
> > >
> > > On Thursday, June 19, 2014 1:29 PM, Bart Kastermans
> <kasterma at kasterma.net> wrote:
> > >
> > >
> > > On 19 Jun 2014, at 13:19, carol white <wht_crl at yahoo.com> wrote:
> > >
> > > > Hi,
> > > > Is there a way to extract a subset of non-contiguous elements of
> a matrix elegantly and with 1 or very few scripts?
> > > >
> > > > Suppose I have a matrix of positive and negative numbers (m) and
> I
> > > > want to retrieve only the positive number. This I can do
> > > >
> > > > which(m>0, arr.ind=T) which gives the indices of positive
> elements like (37,1), (80,1), ..., (54,2) etc. How can I extract
> positive numbers without looping on the indexes provided by which to
> make a new matrix?
> > >
> > > What matrix do you want?  For e.g.
> > >
> > > > tm.1 <- matrix(c(11,22,33,-4), ncol=2)
> > > > which(tm.1 > 0, arr.ind=TRUE)
> > >    row col
> > > [1,]  1  1
> > > [2,]  2  1
> > > [3,]  1  2
> > > > tm.1[which(tm.1 > 0, arr.ind=TRUE)]
> > > [1] 11 22 33
> > >
> > > The extracted values do not form a matrix.
> > >
> > > Either the above contains the answer, or I don’t understand the
> question.
> > >
> > > Best,
> > > Bart
> > >
> > >
> > > >
> > > > thanks,
> > > >
> > > > Carol
> > >
> > >
> >
> >
>         [[alternative HTML version deleted]]
>       [[alternative HTML version deleted]]

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