[R] Subarrays

[Tony Plate]

Here's one way:

 > subarray <- function(x, marginals, intervals) {
+     if (length(marginals) != length(intervals))
+         stop("marginals and intervals must be the same length 
(intervals can be a list)")
+     if (any(marginals<1 | marginals>length(dim(x))))
+         stop("marginals must contain values in 1:length(dim(x))")
+     ic <- Quote(x[, drop=T])
+     # ic has 4 elts with one empty index arg
+     ic2 <- ic[c(1, 2, rep(3, length(dim(x))), 4)]
+     # ic2 has an empty arg for each dim of x
+     ic2[marginals+2] <- intervals
+     eval(ic2)
 > }

 > subarray(v, c(1,4), c(3,2))
      [,1] [,2] [,3] [,4]
[1,]   67   83   99  115
[2,]   71   87  103  119
[3,]   75   91  107  123
[4,]   79   95  111  127
 > subarray(v, c(1,4), list(3,2))
      [,1] [,2] [,3] [,4]
[1,]   67   83   99  115
[2,]   71   87  103  119
[3,]   75   91  107  123
[4,]   79   95  111  127
 > subarray(v, c(1,3,4), list(c(1,3,4),1,2))
      [,1] [,2] [,3] [,4]
[1,]   65   69   73   77
[2,]   67   71   75   79
[3,]   68   72   76   80
 >

Question for language experts: is this the best way to create and 
manipulate R language expressions that contain empty arguments, or are 
there other preferred ways?

-- Tony Plate

Gunnar Hellmund wrote:
> Define an array
> 
> 
>>v<-1:256
>>dim(v)<-rep(4,4)
> 
> 
> Subarrays can be obtained as follows:
> 
> 
>>v[3,2,,2]
> 
> [1]  71  87 103 119
> 
>>v[3,,,2]
> 
>      [,1] [,2] [,3] [,4]
> [1,]   67   83   99  115
> [2,]   71   87  103  119
> [3,]   75   91  107  123
> [4,]   79   95  111  127
> 
> In the general case this procedure is very tedious. 
> 
> Given an array 
> A, dim(A)=(dim_1,dim_2,...,dim_d) 
> and two vectors
> v1=(n_i1,...n_ik), v2=(int_1,...,int_k) ('marginals' and relevant
> 'interval numbers')
> is there a smart way to obtain 
> A[,...,int_1,....,int_2,....,....,int_k,....]
> ?
> 
> Best wishes
> Gunnar Hellmund
> 
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