[R] how to use a matrix as an index to another matrix?
Eric Berger
er|cjberger @end|ng |rom gm@||@com
Fri Oct 11 12:30:30 CEST 2019
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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