> -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-bounces@r- > project.org] On Behalf Of Zhenjiang Lan > Sent: Tuesday, June 12, 2012 8:28 AM > To: r-help@r-project.org > Subject: [R] How to index a matrix with different row-number for each > column? > > here's my question: suppose I have a matrix: > > mt<-matrix(1:12,ncol=6) > > now I have a vector > > vt<-c(1,2,2,2,1,2) > > which means I want to get: > the 1st row for column1; > the 2nd row for column2; > the 2nd row for column3; > the 2nd row for column4; > ... > > that what I want is this vector: > 1,4,6,8,9,12 > > Does anyone know how to do this fast? > I know I can use for-loop to travel all columns,but that's not what I > want. >
There may be a more memory efficient method, but this will get the job done. diag(mt[vt,1:6]) Hope this is helpful, Dan Daniel J. Nordlund Washington State Department of Social and Health Services Planning, Performance, and Accountability Research and Data Analysis Division Olympia, WA 98504-5204 ______________________________________________ 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.
