Thank you! This seems to work!!! Silvia.
Charles C. Berry wrote: > > > See below > > On Wed, 29 Apr 2009, Silvia Lomascolo wrote: > >> >> Hi R community, >> I am trying to obtain a sample from a matrix but sample(my.matrix) >> doesn't >> do what I need. I have a matrix of 1287 interactions between the species >> in >> columns and the species in rows and I want to obtain a smaller matrix >> with >> say, 800 interactions, that may or may not have the same number of >> columns >> and/or rows (i.e., some interactions may not be retrieved in a smaller >> sample). For example, my original mock matrix M is >> >> >> [,1] [,2] [,3] [,4] [,5] >> [1,] 140 100 90 40 20 >> [2,] 126 90 81 36 18 >> [3,] 84 60 54 24 12 >> [4,] 70 50 45 20 10 >> [5,] 42 30 27 12 6 >> >> The command sample(my.matrix) samples whole cells from my matrix, such >> that >> if the interaction between species 1 and 1 is included in the sample, >> they >> always show 140 interactions. But what I want is to sample "cases" >> within >> each cell. My sample matrix S could have =<140 interactions between >> species >> 1 and 1, =<100 between species 1 and 2, etc. Again, if some combination >> is >> absent from the sample matrix, that's OK. >> >> Here's my code, in case it helps: >> >> pla<- c(10, 9, 6, 5, 3) #abundance of pla species >> pol<- c(14, 10, 9, 4, 2) #abundance of pol species >> m<-pla%*%t(pol) #matrix of interactions according to pla and pol >> abundance >> m >> [,1] [,2] [,3] [,4] [,5] >> [1,] 140 100 90 40 20 >> [2,] 126 90 81 36 18 >> [3,] 84 60 54 24 12 >> [4,] 70 50 45 20 10 >> [5,] 42 30 27 12 6 >> >> sample(m) #doesn't give me what I want... >> > > If I understand you, > >> m.index <- 1:length(m) >> # sample all 1287: >> new.m <- matrix(table( factor( sample(rep(m.index,m),1287), m.index >> )),nr=5) >> all.equal(m, new.m) # verify that original was reproduced > [1] TRUE >> # now sample just 800 >> new.m <- matrix(table( factor( sample(rep(m.index,m),800), m.index > )),nr=5) >> m-new.m # look at the difference > > Chuck > > >> I have searched the forum for an answer but all questions regarding >> sampling >> from matrices refer to sampling whole rows or columns, not "cases" within >> a >> matrix. >> Thanks in advance for any help! Silvia. >> -- >> View this message in context: >> http://www.nabble.com/How-do-I-sample-%22cases%22-within-a-matrix--tp23296664p23296664.html >> Sent from the R help mailing list archive at Nabble.com. >> >> ______________________________________________ >> 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. >> > > Charles C. Berry (858) 534-2098 > Dept of Family/Preventive > Medicine > E mailto:cbe...@tajo.ucsd.edu UC San Diego > http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901 > > ______________________________________________ > 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. > > -- View this message in context: http://www.nabble.com/How-do-I-sample-%22cases%22-within-a-matrix--tp23296664p23317180.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ 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.
