[R] how to take multiple subsets from a matrix

[David Winsemius]

On Mar 17, 2010, at 8:54 PM, dc896148 wrote:

>
> useR's
> I have a matrix from which I want to take multiple subsets from,  
> according
> to a particular scheme I will now describe.  The matrix below (mat)  
> is 5x5,
> and I want to take 9 subsets of it, each of dimension 3x3.  The best  
> way to
> explain what the result should look like is with the following:
>> dat <- c(3,6,1,9,12,9,2,10,6,5,3,13,1,4,8,9,4,6,10,11,2,7,3,5,10)
>> miss <- c(2,8,10,16,23)
>> dat[miss] <- NA
>> mat <- matrix(dat,5,5)
>> mat
>     [,1] [,2] [,3] [,4] [,5]
> [1,]    3    9    3   NA    2
> [2,]   NA    2   13    4    7
> [3,]    1   NA    1    6   NA
> [4,]    9    6    4   10    5
> [5,]   12   NA    8   11   10
>
> I define a multidimensional array to hold the 9 subarrays I want:
>> subarray <- array(0,dim=c(3,3,9))
>
> I want to populate each 3x3 array within subarray with the value  
> from mat,
> taking the values from mat to be the center values of each array,
> respectively.  so, excluding the edges of mat, there are 9 values to  
> build
> 3x3 arrays from: 2,13,4,NA,1,6,6,4,10

 > rc <- 2:4
 > cc <- 2:4
 > expand.grid(rc,cc)
   Var1 Var2
1    2    2
2    3    2
3    4    2
4    2    3
5    3    3
6    4    3
7    2    4
8    3    4
9    4    4
 > centers<-expand.grid(rc,cc)
 > arr <- array(, c(3,3,9))
 > arr[ , , 1:9] <-sapply(1:9, function(x)
	mat[(centers$Var1[x]-1):(centers$Var1[x]+1),
		(centers$Var2[x]-1):(centers$Var2[x]+1) ])
 > arr
, , 1

      [,1] [,2] [,3]
[1,]    3    9    3
[2,]   NA    2   13
[3,]    1   NA    1

, , 2

      [,1] [,2] [,3]
[1,]   NA    2   13
[2,]    1   NA    1
[3,]    9    6    4

<snipped>
I suppose you could reverse the roles of Var1 and Var2 to get the  
order you used in your example.

-- 
David.


>
> The output for subarray should look like this:
> [,,1]
>     [,1] [,2] [,3]
> [1,]    3    9    3
> [2,]  NA    2   13
> [3,]    1  NA    1
> [,,2]
>     [,1] [,2] [,3]
> [1,]    9    3   NA
> [2,]    2   13   4
> [3,]  NA    1    6
> [,,3]
>     [,1] [,2] [,3]
> [1,]    3  NA    2
> [2,]   13   4    7
> [3,]    1   6   NA
> ...
> ...
> [,,9]
>     [,1] [,2] [,3]
> [1,]   1    6   NA
> [2,]   4   10    5
> [3,]   8   11  10
>
> I hope this is clear.  Does anyone know a quick and efficient way to  
> do this
> in R?  I was thinking of FOR loops, but as my matrices get very  
> large, this
> process may take some time.  Any ideas?
>
> Thank you in advance
> dxc13
> -- 
> View this message in context: http://n4.nabble.com/how-to-take-multiple-subsets-from-a-matrix-tp1597411p1597411.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org 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.