[R] How do I sample "cases" within a matrix?

[Olivier ETERRADOSSI]

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.
> 

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