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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