# [R] Elegant way to subtract matrix from array

Gabor Grothendieck ggrothendieck at gmail.com
Wed Jul 27 12:08:15 CEST 2011

```On Wed, Jul 27, 2011 at 4:06 AM, steven mosher <moshersteven at gmail.com> wrote:
> there are really two related problems here
>
> I have a 2D matrix
>
>
> A <- matrix(1:100,nrow=20,ncol =5)
>
>
> S <- matrix(1:10,nrow=2,ncol =5)
>
>
> #I want to subtract S from A. so that S would be subtracted from the
> first 2 rows of
>
> #A, then the next two rows and so on.
>
>
> #I have a the same problem with a 3D array
>
> # where I want to subtract Q for every layer (1-10) in Z
>
> # I thought I solved this one with  array(mapply("-",Z,Q),dim=dim(Z))
>
> # but got the wrong answers
>
>
> Z <- array(1:100,dim=c(2,5,10))
>
> Q <- matrix(1:10,nrow=2,ncol =5)
>

For the first one:

matrix(c(t(A)) - c(t(S)), nrow(A), byrow = TRUE)

or this version which may seem a bit more complex but has the
advantage that it shows the general form of which both your questions
are special cases:

ix <- 2:1
aperm(array(c(aperm(A, ix)) - c(t(S)), dim(A)[ix]), ix)

Now, as mentioned, the answer to second question is the same except for ix:

ix <- c(2, 1, 3)
aperm(array(c(aperm(Z, ix)) - c(t(Q)), dim(Z)[ix]), ix)

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```