[R] simple matrix calculation

[Kehl Dániel]

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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