R: optimal way of computing the "product" of two vectors
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Published on 2010-06-14T13:21:33Z
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2010/06/14
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Hi,
Let's assume that I have a vector
r <- rnorm(4)
and a matrix W of dimension 20000*200 for example:
W <- matrix(rnorm(20000*200),20000,200)
I want to compute a new matrix M of dimension 5000*200 such that m11 <- r%*%W[1:4,1], m21 <- r%*%W[5:8,1], m12 <- r%*%W[1:4,2] etc. (i.e. grouping rows 4-by-4 and computing the product).
What's the optimal (speed,memory) way of doing this?
Thanks in advance.
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