[R] how to do pairwise sums in a matrix

From: Gregory Gentlemen <gregory_gentlemen_at_yahoo.ca>
Date: Wed, 21 May 2008 13:22:33 -0400 (EDT)

I am looking for an efficient way to solve the following problem. I have a large matrix of continuous values with a small proportion of missing values. Columns correspond to variables where each variable has two measurements, call them A and B. The matrix is such that the columns are in sequence with respect to the variables. I would like to sum up the two measurements for each variable and each observation (the rows) for the whole matrix. Here is an example of such a matrix with 5 variables (R, S, T, U, V) and 50 observations:

Mat <- matrix(rnorm(500), nrow=50, ncol=10) dimnames(Mat)[[2]] <- c("R.A","R.B","S.A","S.B","T.A","T.B","U.A","U.B","V.A","V.B")

miss.ind <- rbinom(500,1,prob=0.98)
Mat[!as.logical(miss.ind)] <- NA

So I would like to produce 50 by 5 matrix where the columns are the sums of A+B measurements for each of the 5 variables.

Thanks for any help.

Gregory Gentlemen        

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