Re: [R] Split matrix into square submatices
On Fri, Feb 10, 2012 at 12:02:25AM -0800, xiddw wrote:
Hi everybody,
I'm looking for an optimal way to split a big matrix (e.g. ncol = 8,
nrow=8) into small square submatrices (e.g. ncol=2, nrow=2)
For example
If I have
h
[,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
[1,]19 17 25 33 41 49 57
[2,]2 10 18 26 34 42 50 58
[3,]3 11 19 27 35 43 51 59
[4,]4 12 20 28 36 44 52 60
[5,]5 13 21 29 37 45 53 61
[6,]6 14 22 30 38 46 54 62
[7,]7 15 23 31 39 47 55 63
[8,]8 16 24 32 40 48 56 64
and I want to split matriz h into 16 small submatrices:
g[1]
[,1] [,2]
[1,]19
[2,]2 10
g[2]
[,1] [,2]
[1,] 17 25
[2,] 18 26
...
g[4]
[,1] [,2]
[1,] 49 57
[2,] 50 58
...
g[16]
[,1] [,2]
[1,] 55 63
[2,] 56 64
Always the big matrix would be a square matrix and also would have a nrow
and ncol in order of a power of two, so it always could be splitted in at
least halves.
Until now, I'm able to split a matrix but keeping the number of original
matrix columns.
I mean, if I want to split into 16 submatrices, using the following command
what I get its 4 lists:
But I haven't found a way to split resultant lists in order to get 4
matrices for each list.
Hi.
Try the following.
k - 3
n - 2^k
m - 2^(k - 2)
a - matrix(1:n^2, nrow=n, ncol=n)
g - vector(list, length=16)
k - 1
for (j in 0:3) {
for (i in 0:3) {
g[[k]] - a[m*i + 1:m, m*j + 1:m]
k - k + 1
}
}
g[1:5]
[[1]]
[,1] [,2]
[1,]19
[2,]2 10
[[2]]
[,1] [,2]
[1,]3 11
[2,]4 12
[[3]]
[,1] [,2]
[1,]5 13
[2,]6 14
[[4]]
[,1] [,2]
[1,]7 15
[2,]8 16
[[5]]
[,1] [,2]
[1,] 17 25
[2,] 18 26
...
Hope this helps.
Petr Savicky.
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Re: [R] Split matrix into square submatices
On Fri, Feb 10, 2012 at 3:02 AM, xiddw xid...@gmail.com wrote: Hi everybody, I'm looking for an optimal way to split a big matrix (e.g. ncol = 8, nrow=8) into small square submatrices (e.g. ncol=2, nrow=2) For example If I have h [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [1,] 1 9 17 25 33 41 49 57 [2,] 2 10 18 26 34 42 50 58 [3,] 3 11 19 27 35 43 51 59 [4,] 4 12 20 28 36 44 52 60 [5,] 5 13 21 29 37 45 53 61 [6,] 6 14 22 30 38 46 54 62 [7,] 7 15 23 31 39 47 55 63 [8,] 8 16 24 32 40 48 56 64 and I want to split matriz h into 16 small submatrices: g[1] [,1] [,2] [1,] 1 9 [2,] 2 10 g[2] [,1] [,2] [1,] 17 25 [2,] 18 26 ... g[4] [,1] [,2] [1,] 49 57 [2,] 50 58 ... g[16] [,1] [,2] [1,] 55 63 [2,] 56 64 Always the big matrix would be a square matrix and also would have a nrow and ncol in order of a power of two, so it always could be splitted in at least halves. Until now, I'm able to split a matrix but keeping the number of original matrix columns. I mean, if I want to split into 16 submatrices, using the following command what I get its 4 lists: But I haven't found a way to split resultant lists in order to get 4 matrices for each list. g - split(as.data.frame(h), (1:4)) g $`1` V1 V2 V3 V4 V5 V6 V7 V8 1 1 9 17 25 33 41 49 57 5 5 13 21 29 37 45 53 61 $`2` V1 V2 V3 V4 V5 V6 V7 V8 2 2 10 18 26 34 42 50 58 6 6 14 22 30 38 46 54 62 $`3` V1 V2 V3 V4 V5 V6 V7 V8 3 3 11 19 27 35 43 51 59 7 7 15 23 31 39 47 55 63 $`4` V1 V2 V3 V4 V5 V6 V7 V8 4 4 12 20 28 36 44 52 60 8 8 16 24 32 40 48 56 64 Try this: h - matrix(1:64, 8) # input k - kronecker(matrix(1:16, 4, byrow = TRUE), matrix(1, 2, 2)) g - lapply(split(h, k), matrix, nr = 2) -- Statistics Software Consulting GKX Group, GKX Associates Inc. tel: 1-877-GKX-GROUP email: ggrothendieck at gmail.com __ R-help@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.
Re: [R] Split matrix into square submatices
Thanks to Peter and Gabriel for their comments,
The first way which Peter commented was my first approach, however as I use
it for a 'large' matrix (in my case nrow = ncol = 512) and I want to split
it into very small matrices (e.g. nrow = ncol = {4, 8, 16}) both nested
loops I think are a quite expensive and that's why I was looking for a
better way.
The Gabor's approach works perfect for my needs, thanks :)
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