[R] Subsetting a matrix without for-loop

From: Camarda, Carlo Giovanni <Camarda_at_demogr.mpg.de>
Date: Tue 31 Jan 2006 - 02:29:38 EST

Dear R-users,
I'm struggling in R in order to "squeeze" a matrix without using a for-loop.
Although my case is a bit more complex, the following example should help you to understand what I would like to do, but without the slow for-loop.
Thanks in advance,
Carlo Giovanni Camarda

A <- matrix(1:54, ncol=6) # my original matrix A.new <- matrix(nrow=3, ncol=6) # a new matrix which I'll fill # for-loop
for(i in 1:nrow(A.new)){

    B <- A[i:(i+2), ]   # selecting the rows
    C <- apply(B,2,sum) # summing by columns
    A.new[i,] <- C      # inserting in the new matrix

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