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rmatrixelementsamplesequential

in R sampling a list of probabilities returns one position. How do I convert that position, to the element index?

I have a 3x3x3 matrix (MP) of normalized probabilities (sum up to 1). When I run sample(27,1, MP, replace = T), it returns an integer between 1 and 27, presumably the sequential index of the matrix. I would like to know which matrix element in terms of element indexes (eg row number, column number, z number). When building an array of N dimensions (in this case N=3), how does one determine the order of the elements? In other words, if I took an N dimensional array and put all the elements in a list, how can I map the list to the N dimensional elements?

Solution

  • Use which with argument arr.ind=TRUE to return the array indices.

     m <- array((1:27)*10, dim=c(3,3,3))

 x <- sample(27, 1)
 x
 # [1] 20

 which(m == m[x], arr.ind=TRUE)
 #      dim1 dim2 dim3
 # [1,]    2    1    3