Both Oliver's and Chuck's code seem to work. Thank you very much!! I will keep working on this and might get back to the forum for more help if I get stuck. Thanks, really.
PS: Does any of you know why your replies don't show up in the main forum? I get the message that the message hasn't been posted but all of you did get it. I'm subscribed to the list. I followed all the instructions to do it, but it still says that I cannot post on the forum... I'm confused Olivier ETERRADOSSI wrote: > > Hi again Silvia (last time... ?), > now : do I understand : you want > 1) to randomly select some intersections between rows and columns > 2) randomly select a number of cases for each intersection (being <= the > number of initial cases ? > > if yes, here is my solution, using your example : > > # select intersections, put them in new matrix mm > mm<-m[sample(seq(1,5,by=1),2),sample(seq(1,5,by=1),3)] > > # make a function that samples a number of cases > foo<-function(x) sample(seq(1,x,by=1),1) > > # use mapply > mmm<-matrix(mapply(mm,FUN=foo),dim(mm)[1],dim(mm)[2]) > > On my computer it seems to work... hope this really help, this time ! > 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--tp23296664p23317153.html Sent from the R help mailing list archive at Nabble.com. ______________________________________________ [email protected] 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.
