Store your 'matrix' as a data frame. Call it 'SchoolVals' say. Then SchoolMeans <- with(SchoolVals, tapply(value, school, mean))
should do it. If you have missing values you want to ignore: SchoolMeans <- with(SchoolVals, tapply(value, school, mean, na.rm = TRUE)) Bill Venables http://www.cmis.csiro.au/bill.venables/ -----Original Message----- From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On Behalf Of cindy Guo Sent: Monday, 9 February 2009 1:32 PM To: r-help@r-project.org Subject: [R] question about mean Hello, everyone, I have a matrix like following: school value A .1 A .2 A .15 A .2 B .3 B .5 C .3 C .3 C .4 C .5 C .6 C .9 C 1 I want to get the mean 'value' for each 'school', but each school has different length. Is there any way to do this fast? Because my data has hundreds of thousands of rows. Thank you, Cindy [[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. ______________________________________________ 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.
