[R] building a subscript programatically

[Dennis Murphy]

Here's a hack, but perhaps you might want to rethink what type of
output you want.

# Function:
g <- function(arr, lastSubscript = 1) {
   n <- length(dim(arr))
   commas <- paste(rep(',', n - 1), collapse = '')
   .call <- paste('arr[', commas, lastSubscript, ']', sep = '')
   eval(parse(text = .call))
   }

# Examples:
a1 <- array(1:8, c(2, 2, 2))
a2 <- array(1:16, c(2, 2, 2, 2))
a3 <- array(1:32, c(2, 2, 2, 2, 2))
g(a1, 2)
g(a2, 2)
g(a3, 2)

Notice the subscripting in the last two examples - if you only want
one submatrix returned, then try this:

h <- function(arr, lastSubscript = c(1)) {
   n <- length(dim(arr))
   subs <- if(length(lastSubscript) > 1)
              paste(lastSubscript, collapse = ',') else lastSubscript
   .call <- paste('arr[,,', subs, ']', sep = '')
   eval(parse(text = .call))
   }
h(a2, c(1, 1))
h(a3, c(2, 1, 1))

These functions have some ugly code, but I think it does what you were
looking for. Hopefully someone can devise a more elegant solution.

Dennis

HTH

2011/11/1 Ernest Adrogué <nfdisco at gmail.com>:
> Hi,
>
> On ocasion, you need to subscript an array that has an arbitrary
> (ie. not known in advance) number of dimensions. How do you deal with
> these situations?
> It appears that it is not possible use a list as an index, for
> instance this fails:
>
>> x <- array(NA, c(2,2,2))
>> x[list(TRUE,TRUE,2)]
> Error in x[list(TRUE, TRUE, 2)] : invalid subscript type 'list'
>
> The only way I know is using do.call() but it's rather ugly. There
> must be a better way!!
>
>> do.call('[', c(list(x), TRUE, TRUE, 2))
>     [,1] [,2]
> [1,]   NA   NA
> [2,]   NA   NA
>
> Any idea?
>
> Regards,
> Ernest
>
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