Thanks, the drop=FALSE is bound to come in handy. And aggregate was indeed what I was looking.
Thanks again 2011/3/27 David Winsemius <dwinsem...@comcast.net>: > > On Mar 27, 2011, at 3:22 AM, peter dalgaard wrote: > >> >> On Mar 27, 2011, at 08:25 , David Winsemius wrote: >> >>> >>> On Mar 26, 2011, at 10:26 PM, fisken wrote: >>> >>>> I was wondering if it is possible to do the following in a smarter way. >>>> >>>> I want get the mean value across the columns of a matrix, but I want >> >> _along_ the columns, I assume. >> >>>> to do this on subrows of the matrix, given by some vector(same length >>>> as the the number of rows). Something like >>>> >>>> nObs<- 6 >>>> nDim <- 4 >>>> m <- matrix(rnorm(nObs*nDim),ncol=nDim) >>>> fac<-sample(1:(nObs/2),nObs,rep=T) >>>> >>>> ##loop trough different 'factor' levels >>>> for (i in unique(fac)) >>>> print(apply(m[fac==i,],2,mean)) >>> >>> This would be a lot simpler and faster: >>> >>> colMeans(m[unique(fac),]) >>> >>> #[1] 1.3595197 -0.1374411 0.1062527 -0.3897732 >>> >> >> Say what??? (I suspect David needs to get his sleep - or coffee, if he is >> in Europe.) > > At that point it was sleep that I needed. Now .... trying to decide if I > should just go back to bed or make coffee. > >> >> How about: >> >>> aggregate(m,list(fac),mean) >> >> Group.1 V1 V2 V3 V4 >> 1 1 -0.03785420 -0.2573805 -0.3025759 0.006999996 >> 2 2 -1.39961300 0.2296900 -0.1122359 -0.302734531 >> 3 3 0.50886649 0.6546153 -0.4270368 -0.411807709 >>> >>> by(m,list(fac),colMeans) >> >> : 1 >> V1 V2 V3 V4 >> -0.037854195 -0.257380542 -0.302575901 0.006999996 >> ------------------------------------------------------------- >> : 2 >> V1 V2 V3 V4 >> -1.3996130 0.2296900 -0.1122359 -0.3027345 >> ------------------------------------------------------------- >> : 3 >> V1 V2 V3 V4 >> 0.5088665 0.6546153 -0.4270368 -0.4118077 >>> >> >> >> >> -- >> Peter Dalgaard >> > > David Winsemius, MD > West Hartford, CT > > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
