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. ______________________________________________ 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.
