Dear David, Ted, Kjetil, Petr,
thank you, you guys did a great job, I'll use your ideas in the future
for sure.
After I sent the question I figured a way, see below.
x <- 1:81
b <- 1:3
Q <- matrix(x,9,9)
result <- matrix(matrix(colSums(matrix(t(Q),3)),,3,TRUE) %*% b,3,3)
I hope there is no error in this solution and you can use this idea
sometime!
Thank you again, have a great day
Daniel
2012.03.29. 8:48 keltezéssel, Petr Savicky írta:
On Wed, Mar 28, 2012 at 10:46:11PM +0200, Kehl Dániel 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
- Multiply A1*B and get a scalar, the first element of my new 3x3 matrix.
Hi.
Try the following, which is based on the solution by Ted Harding.
# some input
A<- matrix(1:81, nrow=9, ncol=9)
B<- 7:5
# compute the 3 x 3 matrix
C<- diag(3)[rep(1:3, each=3), ]
D<- cbind(rbind(B, 0, 0), rbind(0, B, 0), rbind(0, 0, B))
R1<- D %*% A %*% C
# compare with another approach
E<- A * matrix(B, nrow=9, ncol=9) # component wise product
C<- diag(3)[rep(1:3, each=3), ]
R2<- t(C) %*% E %*% C
max(abs(R1 - R2)) # [1] 0
Hope this helps.
Petr Savicky.
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