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

Bart Kastermans kasterma at kasterma.net
Thu Jun 19 15:07:59 CEST 2014

```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
> >
> >
>
>

```

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