extract a subset of non-contiguous elements of a matrix

Fri, Jun 20, 2014 12:13 AM

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)

[,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 T?gersen; 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 T?gersen
<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


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-------- 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

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

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extract a subset of non-contiguous elements of a matrix

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