Best way to feed which(,arr.ind=T) back into matrix in R?

问题

I have extracted the array indeces of some elements I want to look at as follows:

mat = matrix(0,10,10)

arrInd = which(mat ==0,arr.ind = T)

Then I do some more operations on this matrix and eventually end up with a vector or rows rowInd and a vector of columns colInd. I want us these indeces to insert values into another matrix, say mat2. But I can't seem to figure out a way to do this without looping or doing the modular arithmetic calculation myself. I realize I could take something like

mat2[rowInd*(colInd-1)+rowInd]

in order to transform back to the 1-d indexing. But since R usually has built in functions to do this sort of thing, I was wondering if there is any more concise way to do this? It would just seem natural that such a handy data-manipulation function like which(,arr.ind=T) would have a handy inverse.

EDIT: I tried using mat2[rowInd,colInd], but this did not work.

Best,

Paul

回答1:

Have a read on R intro: indexing a matrix on the use of matrix indexing. which(, arr.ind = TRUE) returns a two column matrix suitable for direct use of matrix indexing. For example:

A <- matrix(c(1L,2L,2L,1L), 2)
iv <- which(A == 1L, arr.ind = TRUE)

#     row col
#[1,]   1   1
#[2,]   2   2

A[iv]
# [1] 1 1

If you have another matrix B which you want to update values according to iv, just do

B[iv] <- replacement

Maybe for some reason you've separated row index and column index into rowInd and colInd. In that case, just use

cbind(rowInd, colInd)

as indexing matrix.

来源：https://stackoverflow.com/questions/40227994/best-way-to-feed-which-arr-ind-t-back-into-matrix-in-r

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matrix

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