see inline.
On Wed, Mar 28, 2012 at 2:46 PM, Kehl Dániel <ke...@ktk.pte.hu> wrote:
> Dear list-members,
>
> I have a 9-by-9 matrix lets call it A with first row a11, a12, a13,..., a19
> etc.
> I also have a vector of length 3 (B).
> I want to construct a matrix of size 3x3 in the following way:
> - divide matrix A to 9 3x3 blocks
> - first is
> a11, a12, a13
> a21, a22, a23
> a31, a32, a33
> - I want to get rowSums of this A1 matrix
A %*% C, where C is a 9x3 matrix
A <- matrix(1:81, 9, 9)
> ones <- rep(1, 3)
> zeros <- rep(0, 3)
> C <- rbind(cbind(ones, zeros, zeros), cbind(zeros, ones, zeros), cbind(zeros,
> zeros, ones))
> C
ones zeros zeros
[1,] 1 0 0
[2,] 1 0 0
[3,] 1 0 0
[4,] 0 1 0
[5,] 0 1 0
[6,] 0 1 0
[7,] 0 0 1
[8,] 0 0 1
[9,] 0 0 1
> A %*%C
ones zeros zeros
[1,] 30 111 192
[2,] 33 114 195
[3,] 36 117 198
[4,] 39 120 201
[5,] 42 123 204
[6,] 45 126 207
[7,] 48 129 210
[8,] 51 132 213
[9,] 54 135 216
> - Multiply A1*B and get a scalar, the first element of my new 3x3 matrix.
But from your description I get the new matrix will be 9x3, as above?
> B <- 1:3
> A %*%C %*% B
[,1]
[1,] 828
[2,] 846
[3,] 864
[4,] 882
[5,] 900
[6,] 918
[7,] 936
[8,] 954
[9,] 972
> (A %*%C %*% B)[1]
[1] 828
Kjetil
>
> I could do that with loop. Can you suggest something that is more elegant
> and faster?
>
> Thank you
> Daniel
>
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