[R] converting matrix in array

[David Winsemius]

On Aug 28, 2011, at 5:09 PM, marco wrote:

> Hi everyone,
> have a small problem trying to converting a dataset in matrix form  
> to an
> array.
> Specifically: data include 3D measurement -x,y,z of 59 points in 36  
> objects.
> They are stored as a matrix (x) of 2124 rows and 3 columns.
> What I want to do is to extract each subject's dataset using an  
> array (b).
> Accordingly, I tried the following command:
>
> b<-array(a,c(59,3,36)).
>
> The problem is that the resulting array for some strange reason  
> change the
> order of the original data.

Not for a "strange reason". The 59 elements in b in column-1 would be  
the same in both , but you don't seem to realize that the 60th through  
the 118th elements in your original matrix were also all in the x- 
column. You have now effectively moved those values to what you were  
thinking to use as the y-column.

> Specifically, I noticed that, for example, in the first subject, the  
> x y z
> values of the first point are the x values of the first 3 subjects.

You should have kept the last dimension the same and redimension the  
object/points indexing by factoring the 2124 rows into what will  
become columns and row and leave the xyz coordinates "at the end". I  
think that redimensioning as b<-array(a,c(59, 36, 3)) would have re- 
folded it sensiblyly. So the b[n,m,] call would retrieve xyz- 
coordinates from the n-th point of the m-th object.



>
> Did I perhaps missed something?
> Thanks in advance
>
>
>
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David Winsemius, MD
West Hartford, CT