[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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