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

Bert Gunter bgunter@4567 @end|ng |rom gm@||@com
Mon Oct 28 22:22:23 CET 2019

```No loops necessary. Use array indexing (see ?"[", of course -- the section
on matrices and arrays)

set.seed(123)
A <- matrix(sample(1:10), nrow = 5)
B <- matrix(c(sample(1:5), sample(1:5)), nrow =5, byrow = FALSE)
## The following could be a 1-liner, but I broke it out for clarity.
ix <- cbind(as.vector(B), rep(1:2, e=5))
ix
matrix(A[ix], ncol =2)

Cheers,
Bert

Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Mon, Oct 28, 2019 at 1:48 PM Linus Chen <linus.l.chen using gmail.com> wrote:

> Hi Jinsong,
>
> In such a case I think explicit loop IS the most elegant solution.
> for(i in 1:2) A[,i] <- A[,i][B[,i]]
>
> Linus
>
> On Fri, 11 Oct 2019 at 11:44, 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]]
> >
> >
> > PS., I know how to do the above thing by loop.
> >
> > Best,
> > Jinsong
> >
> > ______________________________________________
> > R-help using r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> http://www.R-project.org/posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
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