Well Silvia, I understand that I didn't read your post carefully. Forget about my previous (unrelevant) post. regards. olivier
Olivier ETERRADOSSI wrote: > > Hi Silvia, > excuse me if I'm missing something but sampling the row numbers should > make it : > > let M be your matrix, and spM a sample with n rows : > spM <- M[sample(seq(1,dim(M)[1],by=1), n),] > > Regards, Olivier > > > 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... >> >> 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--tp23296664p23315275.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.
