18:21:24
-0700> From: [EMAIL PROTECTED]> To: [EMAIL PROTECTED]> Subject: Re: [R]
Computing row means for sets of 2 columns> CC: [EMAIL PROTECTED]> > m <-
matrix(1:40, ncol=4);> groups <- rep(1:2, each=2);> uGroups <- unique(groups);>
mMeans <- matrix(NA,
m <- matrix(1:40, ncol=4);
groups <- rep(1:2, each=2);
uGroups <- unique(groups);
mMeans <- matrix(NA, nrow=nrow(m), ncol=length(uGroups));
for (gg in seq(along=uGroups)) {
mMeans[,gg] <- rowMeans(m[,groups == uGroups[gg], drop=FALSE]);
}
(Preallocation of result matrix is more memory efficient
Is there a better or more efficent approach than this without the use of t() ?
> (m <- matrix(1:40, ncol=4)) [,1] [,2] [,3] [,4] [1,]1 11 21 31
> [2,]2 12 22 32 [3,]3 13 23 33 [4,]4 14 24 34
> [5,]5 15 25 35 [6,]6 16 26 36 [7,]
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