How to subset a matrix?

[LosemindL]

Hi all,

Lets say I have a matrix A which is m x n.

I also have a mask matrix MASK which is m x n with values in T/F, where T
values make a sub-matrix in regutangular shape...

I applied B=A[MASK] and it didn't work as expected...

Any thoughts?

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[Oliver Ruebenacker]

     Hello,

On Tue, Jun 12, 2012 at 12:44 PM, Michael <[hidden email]> wrote:

> Hi all,
>
> Lets say I have a matrix A which is m x n.
>
> I also have a mask matrix MASK which is m x n with values in T/F, where T
> values make a sub-matrix in regutangular shape...
>
> I applied B=A[MASK] and it didn't work as expected...
>
> Any thoughts?

  What did you expect and what happened?

  I'm assuming you did:

  B <- A[MASK]

  and I would expect the the result would be a vector whose size is
the number of elements in MASK that is TRUE. To get a matrix, you then
set the dim attribute (but I didn't test that).

     Take care
     Oliver

--
Oliver Ruebenacker
Bioinformatics Consultant (http://www.knowomics.com/wiki/Oliver_Ruebenacker)
Knowomics, The Bioinformatics Network (http://www.knowomics.com)
SBPAX: Turning Bio Knowledge into Math Models (http://www.sbpax.org)

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[Sarah Goslee]

Providing a reproducible example is a good idea, but what about this:

> A <- matrix(1:20, nrow=4)
> MASK <- matrix(FALSE, nrow=4, ncol=5)
> MASK[2:3, 1:3] <- TRUE
> MASK
      [,1]  [,2]  [,3]  [,4]  [,5]
[1,] FALSE FALSE FALSE FALSE FALSE
[2,]  TRUE  TRUE  TRUE FALSE FALSE
[3,]  TRUE  TRUE  TRUE FALSE FALSE
[4,] FALSE FALSE FALSE FALSE FALSE
> A[rowSums(MASK) > 0, colSums(MASK) > 0]
     [,1] [,2] [,3]
[1,]    2    6   10
[2,]    3    7   11

Alternatively, you could take the vector returned by:
> A[MASK]
[1]  2  3  6  7 10 11

and put it back into matrix format.

Sarah

On Tue, Jun 12, 2012 at 12:44 PM, Michael <[hidden email]> wrote:

> Hi all,
>
> Lets say I have a matrix A which is m x n.
>
> I also have a mask matrix MASK which is m x n with values in T/F, where T
> values make a sub-matrix in regutangular shape...
>
> I applied B=A[MASK] and it didn't work as expected...
>
> Any thoughts?

--
Sarah Goslee
http://www.functionaldiversity.org

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[Petr Savicky]

On Tue, Jun 12, 2012 at 11:44:51AM -0500, Michael wrote:
> Hi all,
>
> Lets say I have a matrix A which is m x n.
>
> I also have a mask matrix MASK which is m x n with values in T/F, where T
> values make a sub-matrix in regutangular shape...
>
> I applied B=A[MASK] and it didn't work as expected...

Hi.

Try the following.

  A <- matrix(1:16, nrow=4, ncol=4)
  B <- matrix(FALSE, nrow=4, ncol=4)
  B[2:3, 2:3] <- TRUE
  A[rowSums(B) != 0, colSums(B) != 0]

       [,1] [,2]
  [1,]    6   10
  [2,]    7   11

Hope this helps.

Petr Savicky.

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[David Winsemius]

On Jun 12, 2012, at 12:44 PM, Michael wrote:

> Hi all,
>
> Lets say I have a matrix A which is m x n.
>
> I also have a mask matrix MASK which is m x n with values in T/F,  
> where T
> values make a sub-matrix in regutangular shape...
>
> I applied B=A[MASK] and it didn't work as expected...


Perhaps: (not fully tested in absence of reproducible example):

B=A[MASK]
MASK <- matrix(as.logical( c(0,0,0,0,0,
                        0,1,1,1,0,
                        0,1,1,1,0,
                        0,0,0,0,0,
                        0,0,0,0,0)), 5, byrow=TRUE)
#----

      row col
[1,]   2   2
[2,]   3   2
[3,]   2   3
[4,]   3   3
[5,]   2   4
[6,]   3   4
#-----
B <- matrix(B, nrow=1+diff(range(wM[,"row"])),
                ncol=1+diff(range(wM[,"col"])) )

--
David Winsemius, MD
West Hartford, CT

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[David Winsemius]

On Jun 12, 2012, at 1:58 PM, David Winsemius wrote:

>
> On Jun 12, 2012, at 12:44 PM, Michael wrote:
>
>> Hi all,
>>
>> Lets say I have a matrix A which is m x n.
>>
>> I also have a mask matrix MASK which is m x n with values in T/F,  
>> where T
>> values make a sub-matrix in regutangular shape...
>>
>> I applied B=A[MASK] and it didn't work as expected...
>
>
> Perhaps: (not fully tested in absence of reproducible example):
>
> B=A[MASK]
> MASK <- matrix(as.logical( c(0,0,0,0,0,
>                       0,1,1,1,0,
>                       0,1,1,1,0,
>                       0,0,0,0,0,
>                       0,0,0,0,0)), 5, byrow=TRUE)

I see I failed to transpose the line that created wM

wM <- which(MASK, arr.ind=TRUE)

> #----
>
>     row col
> [1,]   2   2
> [2,]   3   2
> [3,]   2   3
> [4,]   3   3
> [5,]   2   4
> [6,]   3   4
> #-----
> B <- matrix(B, nrow=1+diff(range(wM[,"row"])),
>               ncol=1+diff(range(wM[,"col"])) )
>
> --
> David Winsemius, MD
> West Hartford, CT
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

David Winsemius, MD
West Hartford, CT

______________________________________________
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https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.