[R] Efficient way for multiplying vectors with a only certain number of rows in a matrix

[Ralph79]

Dear R-users,

I am working on a problem that I am currently not able to solve efficiently.
It is about multiplying one column of a matrix with only a certain number of
rows of another matrix.

Let me illustrate my problem with an example:

n.obs = 800
n.rowsperobs = 300
n.param = 23

Designmat = matrix(rnorm(n.obs*n.rowsperobs*n.param),ncol=n.param)

Betamat = matrix(rnorm(n.obs*n.param),nrow=n.param)

In this example, "Designmat" consists of 800*300 rows, meaning that 300 rows
belong to one of the 800 observations.

Each observation has also one parameter vector, which is one column in the
"Betamat"-matrix (i.e. Betamat contains the n.obs parameter vectors). 

My goal is to multiply the parameter vector of each observation (i.e. the
respective column in Betamat) with ONLY THOSE ROWS IN DESIGNMAT that belong
to this observation. 
Applied to the example above: The first column in Betamat has to be
multiplied with the first 300 rows in Designmat, the second column in
Betamat has to be multiplied with rows 301 to 600 in Betamat and so on.
Hence, the result of this operation should be a vector of length 240000. 

I can think of solutions implying several loops and/or lapplys, but I guess
that there might be a much easyer and above all faster solution. 

Thank you very much for your help in advance.

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