Reshaping an array - how does it work in R
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 ********************** "The contents of this message do not reflect any position of the U.S. Government or NOAA." ********************** Roy Mendelssohn Supervisory Operations Research Analyst NOAA/NMFS Environmental Research Division Southwest Fisheries Science Center ***Note new address and phone*** 110 Shaffer Road Santa Cruz, CA 95060 Phone: (831)-420-3666 Fax: (831) 420-3980 e-mail: Roy.Mendelssohn at noaa.gov www: http://www.pfeg.noaa.gov/ "Old age and treachery will overcome youth and skill." "From those who have been given much, much will be expected" "the arc of the moral universe is long, but it bends toward justice" -MLK Jr.
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