Thank you, David Here is an example :
Z=matrix(rnorm(20), nrow=4) index=replicate(4, sample(1:5, 3)) P=4 tmpr=list() for (i in 1:P) { tmp = Z[i,index[,i]] tmpr[[i]]=tmp } So, I am trying to pull out the values in Z for each row, but now I only want to pull out the values indexed by the matrix "index" And, since I have large P, so I would like to avoid loop. Any quick way to do this ? Thanks again!! Carrie On Thu, Feb 4, 2010 at 10:39 PM, David Winsemius <dwinsem...@comcast.net>wrote: > > On Feb 4, 2010, at 9:51 PM, Carrie Li wrote: > > Dear R-helpers , >> >> I have a simple loop as follows, however, to be more efficient, I would >> like >> to use any apply functions (tapply, I suppose) >> But how can I do this ? I am not very clear about this. >> >> # Z is a P * Q matrix >> # so for each row of Z, I would like to pull out only some of the >> elements, >> and save as a separate matrix under a list >> # index is a vector with length smaller than Q, but the index could be >> different for each row of Z. >> > > An example would help. Sounds like you need a list structure to hold the > index and I don't see that you have constructed one. > > >> for (i in 1:P) >> { >> tmp = matrix(Z[i, index], nrow=1) >> > > Seems as though you would need to "index" in a verb sense (with "i") the > index (in a noun sense) structure. Might be better to call it "idx". > Perhaos > > tmp = matrix(Z[i, index[[i]] ], nrow=1) # the nrow=1 appears superfluous > > > tmpr[[i]]=tmp >> } >> >> > any help is highly appreciated!! >> >> Thank you all >> >> Carrie >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> 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. >> > > David Winsemius, MD > Heritage Laboratories > West Hartford, CT > > [[alternative HTML version deleted]] ______________________________________________ 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.