[R] Extracting the dimnames of an array with variable dimensions

[Pierre Roudier]

Hi Dennis,

Thanks for your answer, it works very well - clever way to sort the problem!

Cheers,

Pierre

2011/5/16 Dennis Murphy <djmuser at gmail.com>:
> Hi:
>
> Does it have to be an array?  If all you're interested in is the
> dimnames, how about this?
>
> library(plyr)
> foo <- array(data = rnorm(32), dim = c(4,4,2),
>                   dimnames=list(letters[1:4], LETTERS[1:4], letters[5:6]))
>> foo
> , , e
>
>           A          B          C          D
> a -0.2183877 -0.8912908 -2.0175612 -0.8080548
> b  0.4870784 -0.8626293 -0.5641368 -0.5219722
> c  0.8821044  0.3187850  1.2203297 -0.3151186
> d -0.9894656 -1.1779108  0.9853935  0.3560747
>
> , , f
>
>           A          B          C          D
> a  0.7357773 -1.7591637  1.6320887  1.2248529
> b  0.4662315  0.1131432 -0.9790887 -0.6575306
> c -0.3564725 -0.9202688  0.1017894  0.7382683
> d  0.2825117  0.9242299  0.3577063 -1.3297339
>
> # flatten array into a data frame with dimnames as factors
> # adply() converts an array to a data frame, applying a function
> # along the stated dimensions
>  u <- adply(foo, c(1, 2, 3), as.vector)
> subset(u, V1 > 0)[, 1:3]
>   X1 X2 X3
> 2   b  A  e
> 3   c  A  e
> 7   c  B  e
> 11  c  C  e
> 12  d  C  e
> 16  d  D  e
> 17  a  A  f
> 18  b  A  f
> 20  d  A  f
> 22  b  B  f
> 24  d  B  f
> 25  a  C  f
> 27  c  C  f
> 28  d  C  f
> 29  a  D  f
> 31  c  D  f
>
> HTH,
> Dennis
>
> On Sun, May 15, 2011 at 9:20 PM, Pierre Roudier
> <pierre.roudier at gmail.com> wrote:
>> Hi list,
>>
>> In a function I am writing, I need to extract the dimension names of
>> an array. I know this can be acheived easily using dimnames() but my
>> problem is that I want my function to be robust when the number of
>> dimensions varies. Consider the following case:
>>
>> foo <- array(data = rnorm(32), dim = c(4,4,2),
>> dimnames=list(letters[1:4], LETTERS[1:4], letters[5:6]))
>>
>> # What I want is to extract the *names of the dimensions* for which
>> foo have positive values:
>> ind <- which(foo > 0, arr.ind = TRUE)
>>
>> # A first solution is:
>> t(apply(ind, 1, function(x) unlist(dimnames(foo[x[1], x[2], x[3],
>> drop=FALSE]))))
>> # But it does require to know the dimensions of foo
>>
>> I would like to do something like:
>>
>> ind <- which(foo > 0, arr.ind = TRUE)
>> t(apply(ind, 1, function(x) unlist(dimnames(foo[x, drop=FALSE]))))
>>
>> but in that case the dimnames are dropped.
>>
>> Any suggestion?
>>
>> Cheers,
>>
>> Pierre
>>
>> --
>> Scientist
>> Landcare Research, New Zealand
>>
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>>
>



-- 
Scientist
Landcare Research, New Zealand