[R] General indexing in multidimensional arrays

[David Winsemius]

On Aug 1, 2011, at 10:50 AM, Duncan Murdoch wrote:

> On 11-08-01 5:38 AM, Jannis wrote:
>> Dear R community,
>>
>>
>> I have a general question regarding indexing in multidiemensional  
>> arrays.
>>
>> Imagine I have a three dimensional array and I only want to extract  
>> on
>> vector along a single dimension from it:
>>
>>
>> data<- array(rnorm(64),dim=c(4,4,4))
>>
>> result<- data[1,1,]
>>
>> If I want to extract more than one of these vectors, it would now  
>> really
>> help me to supply a logical matrix of the size of the first two  
>> dimensions:
>>
>>
>> indices<- matrix(FALSE,ncol=4,nrow=4)
>> indices[1,3]<- TRUE
>> indices[4,1]<- TRUE
>>
>> result<- data[indices,]

Is this the right answer?

 > result<- which(indices, arr.ind=TRUE)
 > result
      row col
[1,]   4   1
[2,]   1   3

 > apply(result, 1, function(x) data[x[1], x[2], ])
             [,1]       [,2]
[1,]  1.62880528  0.7781005
[2,] -0.08861725 -2.1791674
[3,]  0.78242531 -1.0352826
[4,]  1.40012118 -1.2541230

....if so, it should be possible to encapsulate that behavior in a  
function.


-- 
David Winsemius, MD
West Hartford, CT

>>
>> This, however would give me an error. I am used to this kind of  
>> indexing
>> from Matlab and was wonderingt whether there exists an easy way to do
>> this in R without supplying complicated index matrices of all three
>> dimensions or logical vectors of the size of the whole matrix?
>>
>> The only way I could imagine would be to:
>>
>> result<- data[rep(as.vector(indices),times=4)]
>>
>> but this seems rather complicated and also depends on the order of  
>> the
>> dimensions I want to extract.
>>
>>
>> I do not want R to copy Matlabs behaviour, I am just wondering  
>> whether I
>> missed one concept of indexing in R?
>>
>
> Base R doesn't have anything like that as far as I know. The closest  
> is matrix indexing: you construct a 3 column matrix whose rows are  
> the indices of each element you want to extract.

>
> Possibly plyr or some other package has functions to do this.