[R] how to use a matrix as an index to another matrix?

[Martin Morgan]

A matrix can be subset by another 2-column matrix, where the first column is the row index and the second column the column index. So

idx = matrix(c(B, col(B)), ncol = 2)
A[] <- A[idx]

Martin Morgan

On 10/11/19, 6:31 AM, "R-help on behalf of Eric Berger" <r-help-bounces using r-project.org on behalf of ericjberger using gmail.com> wrote:

    Here is one way
    A <- sapply(1:ncol(A), function(i) {A[,i][B[,i]]})
    
    
    On Fri, Oct 11, 2019 at 12:44 PM Jinsong Zhao <jszhao using yeah.net> wrote:
    
    > Hi there,
    >
    > I have two matrices, A and B. The columns of B is the index of the
    > corresponding columns of A. I hope to rearrange of A by B. A minimal
    > example is following:
    >
    >  > set.seed(123)
    >  > A <- matrix(sample(1:10), nrow = 5)
    >  > B <- matrix(c(sample(1:5), sample(1:5)), nrow =5, byrow = FALSE)
    >  > A
    >       [,1] [,2]
    > [1,]    3    9
    > [2,]   10    1
    > [3,]    2    7
    > [4,]    8    5
    > [5,]    6    4
    >  > B
    >       [,1] [,2]
    > [1,]    2    1
    > [2,]    3    4
    > [3,]    1    5
    > [4,]    4    3
    > [5,]    5    2
    >  > A[,1] <- A[,1][B[,1]]
    >  > A[,2] <- A[,2][B[,2]]
    >  > A
    >       [,1] [,2]
    > [1,]   10    9
    > [2,]    2    5
    > [3,]    3    4
    > [4,]    8    7
    > [5,]    6    1
    >
    > My question is whether there is any elegant or generalized way to replace:
    >
    >  > A[,1] <- A[,1][B[,1]]
    >  > A[,2] <- A[,2][B[,2]]
    >
    > Thanks in advance.
    >
    > PS., I know how to do the above thing by loop.
    >
    > Best,
    > Jinsong
    >
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