How do I sample "cases" within a matrix?

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.