On 29-03-2012, at 13:16, Kehl Dániel wrote: > 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! >
Have you compared result with R1 and R2, provided by Petr's solution? Berend > 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. >> >> ______________________________________________ >> 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. > > ______________________________________________ > 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. > ______________________________________________ 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.
