R: subsetting N-dimensional arrays

Question

Consider the following 3-dimensional array:

set.seed(123)
arr = array(sample(c(1:10)), dim=c(3,4,2))

which yields

> arr
, , 1

     [,1] [,2] [,3] [,4]
[1,]   10    9    8    2
[2,]    5    1    4   10
[3,]    6    7    3    5

, , 2

     [,1] [,2] [,3] [,4]
[1,]    6    7    3    5
[2,]    9    8    2    6
[3,]    1    4   10    9

I'd like to subset it like

arr[c(1,2), c(2,4), c(1)]

but the catch is that I don't know (a) which indices or (b) which dimension the indices are.

What is the best way to access an N-dimensional array with index variables?

ll = list(c(1,2), c(2,4), c(1))

arr[ll]              # doesn't work
arr[grid.expand(ll)] # doesn't work
# ..what else?

Ricardo Saporta · Accepted Answer · 2014-01-29T22:26:47.980

3

use do.call, such as:

do.call(`[`, c(list(arr), ll))

or more cleanly, using a wrapper function:

getArr <- function(...) 
   `[`(arr, ...)

do.call(getArr, ll)

     [,1] [,2]
[1,]   10    5
[2,]    7    3

edited Jan 29 '14 at 22:26

answered Jan 29 '14 at 21:39

Ricardo Saporta

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There is one more thing: how can I assign a value to the subset? – Michael Schubert Jan 29 '14 at 22:27
2

`arr <- do.call("[<-", c(list(arr), ll, 100))` – flodel Jan 30 '14 at 00:06

score 1 · Answer 2 · edited May 23 '17 at 12:31

1

There is the asub function from the abind package:

library(abind)
asub(arr, ll)

which can also do a lot more, in particular extract along a subset of the dimensions (https://stackoverflow.com/a/17752012/1201032). Worth having in your toolbox.

edited May 23 '17 at 12:31

Community

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answered Jan 29 '14 at 23:52

flodel

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`asub` is very nice, I didn't know about that one. Do you know if it is possible to assign values to a selection? – Michael Schubert Jan 30 '14 at 00:08
There is unfortunately no `asub<-` so if that's what you were really after, I'd probably recommend what I commented under ricardo's answer. – flodel Jan 30 '14 at 00:11
There is also afill(), as documented here: http://cran.fhcrc.org/web/packages/abind/index.html – Michael Schubert Jan 30 '14 at 15:52

R: subsetting N-dimensional arrays

2 Answers2

or more cleanly, using a wrapper function:

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