CODEWITHSUNDEEP
rmultidimensional-array
user1766394
user1766394

Reputation: 303

Finding dimensional index in a multi-dimensional array in R

Am looking at say 3-dimensional array M: M<-dim(3,3,3)

I want to find an efficient way to populate M with the following rule: M[i,j,k] = i/10 + j^2 + sqrt(k), ideally without having to write a loop with a for statemenet.

For clarification, there is a simple way to accomplishing this if M were 2-dimensional. If i wanted to have M[i,j] = i/10 + j^2, then i could just do M<-row(M)/10 + col(M)*col(M)

Is there something equivalent for 3-or-higher dimensional arrays?

Upvotes: 6

Views: 1510

Answers (4)

Ben Bolker
Ben Bolker

Reputation: 226162

@James's answer is better, but I think the narrow answer to your question (multidimensional equivalent of row()/col()) is slice.index ...

M<- array(dim=c(3,3,3))
slice.index(M,1)/10+slice.index(M,2)^2+sqrt(slice.index(M,3))

It would be a good idea if someone (I or someone else) posted a suggestion on the r-devel list to make slice.index a "See also" entry on ?row/?col ...

Alternatively (similar to @flodel's new answer):

d <- do.call(expand.grid,lapply(dim(M),seq)) ## create data.frame of indices
v <- with(d,Var1/10+Var2^2+sqrt(Var3))       ## default names Var1, ... Varn 
dim(v) <- dim(M)                             ## reshape into array

Upvotes: 7

flodel
flodel

Reputation: 89057

You can also use arrayInd:

M   <- array(dim = c(3, 3, 3))
foo <- function(dim1, dim2, dim3) dim1/10 + dim2^2 + sqrt(dim3)
idx <- arrayInd(seq_along(M), dim(M), useNames = TRUE)
M[] <- do.call(foo, as.data.frame(idx))

I feel this approach has potential for less typing as the number of dimensions increases.

Upvotes: 2

IRTFM
IRTFM

Reputation: 263332

Doing it from the "ground up" so to speak.

 i <- rep(1:3, times=3*3)
 j <- rep(1:3 , times= 3, each=3)
 k <- rep(1:3 , each= 3*3)
 M <- array( i/10 + j^2 + sqrt(k), c(3, 3, 3))
 M

Upvotes: 1

James
James

Reputation: 66834

How about using nested outers?

outer(1:3/10,outer((1:3)^2,sqrt(1:3),"+"),"+")
, , 1

     [,1] [,2] [,3]
[1,]  2.1  5.1 10.1
[2,]  2.2  5.2 10.2
[3,]  2.3  5.3 10.3

, , 2

         [,1]     [,2]     [,3]
[1,] 2.514214 5.514214 10.51421
[2,] 2.614214 5.614214 10.61421
[3,] 2.714214 5.714214 10.71421

, , 3

         [,1]     [,2]     [,3]
[1,] 2.832051 5.832051 10.83205
[2,] 2.932051 5.932051 10.93205
[3,] 3.032051 6.032051 11.03205

Upvotes: 4

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