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
> --
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>
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