[R] Reshaping an array - how does it work in R

[Bert Gunter]

arrays are vectors stored in column major order.  So the answer is: reindexing.

Does this make it clear:

> v <- array(1:24,dim=2:4)
> as.vector(v)
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

> v
, , 1

     [,1] [,2] [,3]
[1,]    1    3    5
[2,]    2    4    6

, , 2

     [,1] [,2] [,3]
[1,]    7    9   11
[2,]    8   10   12

, , 3

     [,1] [,2] [,3]
[1,]   13   15   17
[2,]   14   16   18

, , 4

     [,1] [,2] [,3]
[1,]   19   21   23
[2,]   20   22   24

> w <- array(as.vector(v),dim=c(6,4)) ## you would use v instead of w for the assignment
> w
     [,1] [,2] [,3] [,4]
[1,]    1    7   13   19
[2,]    2    8   14   20
[3,]    3    9   15   21
[4,]    4   10   16   22
[5,]    5   11   17   23
[6,]    6   12   18   24
> identical(as.vector(w), as.vector(v))
[1] TRUE


However copying may occur anyway as part of R's semantics. Others will
have to help you on that, as the details here are beyond me.

Cheers,
Bert



Cheers,
Bert


Bert Gunter

"The trouble with having an open mind is that people keep coming along
and sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )


On Fri, Mar 18, 2016 at 2:28 PM, Roy Mendelssohn - NOAA Federal
<roy.mendelssohn at noaa.gov> wrote:
> Hi All:
>
> I am working with a very large array.  if noLat is the number of latitudes, noLon the number of longitudes and noTime the number of  time periods, the array is of the form:
>
> myData[noLat, no Lon, noTime].
>
> It is read in this way because that is how it is stored in a (series) of netcdf files.  For the analysis I need to do, I need instead the array:
>
> myData[noLat*noLon, noTime].  Normally this would be easy:
>
> myData<- array(myData,dim=c(noLat*noLon,noTime))
>
> My question is how does this command work in R - does it make a copy of the existing array, with different indices for the dimensions, or does it just redo the indices and leave the given array as is?  The reason for this question is my array is 30GB in memory, and I don’t have enough space to have a copy of the array in memory.  If the latter I will have to figure out a work around to bring in only part of the data at a time and put it into the proper locations.
>
> Thanks,
>
> -Roy
>
>
>
> **********************
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> **********************
> Roy Mendelssohn
> Supervisory Operations Research Analyst
> NOAA/NMFS
> Environmental Research Division
> Southwest Fisheries Science Center
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>
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