How do I sample "cases" within a matrix?
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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