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

Olivier ETERRADOSSI olivier.eterradossi at ema.fr
Thu Apr 30 14:00:39 CEST 2009

```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.
>

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