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

Dénes Tóth toth.denes at ttk.mta.hu
Fri Mar 18 22:56:23 CET 2016

Hi Roy,

R (usually) makes a copy if the dimensionality of an array is modified, 
even if you use this syntax:
x <- array(1:24, c(2, 3, 4))
dim(x) <- c(6, 4)

See also ?tracemem, ?data.table::address, ?pryr::address and other tools 
to trace if an internal copy is done.

Workaround: use data.table::setattr or bit::setattr to modify the 
dimensions in place (i.e., without making a copy). Risk: if you modify 
an object by reference, all other objects which point to the same memory 
address will be modified silently, too.


On 03/18/2016 10:28 PM, Roy Mendelssohn - NOAA Federal 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
> **********************
> "The contents of this message do not reflect any position of the U.S. Government or NOAA."
> **********************
> Roy Mendelssohn
> Supervisory Operations Research Analyst
> Environmental Research Division
> Southwest Fisheries Science Center
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