[R] Aggregate matrix in a 2 by 2 manor

[Jeff Newmiller]

An alternative (more compact, not necessarily faster, because apply is still a for loop inside):

f <- function( m, nx, ny ) {
  # redefine the dimensions of my
  a <- array( m
             , dim = c( ny
                    , nrow( m ) %/% ny
                    , ncol( m ) %/% nx )
            )
  # apply mean over dim 1
  apply( a, c( 2, 3 ), FUN=mean )
}
f( tst, nx, ny )
                    
-- 
Sent from my phone. Please excuse my brevity.

On July 27, 2016 9:08:32 AM PDT, David L Carlson <dcarlson at tamu.edu> wrote:
>This should be faster. It uses apply() across the blocks. 
>
>> ilon <- seq(1,8,nx)
>> ilat <- seq(1,4,ny)
>> cells <- as.matrix(expand.grid(ilat, ilon))
>> blocks <- apply(cells, 1, function(x) tst[x[1]:(x[1]+1),
>x[2]:(x[2]+1)])
>> block.means <- colMeans(blocks)
>> tst_2x2 <- matrix(block.means, 2, 4)
>> tst_2x2
>     [,1] [,2] [,3] [,4]
>[1,]  3.5 11.5 19.5 27.5
>[2,]  5.5 13.5 21.5 29.5
>
>-------------------------------------
>David L Carlson
>Department of Anthropology
>Texas A&M University
>College Station, TX 77840-4352
>
>
>
>-----Original Message-----
>From: R-help [mailto:r-help-bounces at r-poject.org] On Behalf Of Anthoni,
>Peter (IMK)
>Sent: Wednesday, July 27, 2016 6:14 AM
>To: r-help at r-project.org
>Subject: [R] Aggregate matrix in a 2 by 2 manor
>
>Hi all,
>
>I need to aggregate some matrix data (1440x720) to a lower dimension
>(720x360) for lots of years and variables
>
>I can do double for loop, but that will be slow. Anybody know a quicker
>way?
>
>here an example with a smaller matrix size:
>
>tst=matrix(1:(8*4),ncol=8,nrow=4)
>tst_2x2=matrix(NA,ncol=4,nrow=2)
>nx=2
>ny=2
>for(ilon in seq(1,8,nx)) {
>  for (ilat in seq(1,4,ny)) {
>    ilon_2x2=1+(ilon-1)/nx
>    ilat_2x2=1+(ilat-1)/ny
>    tst_2x2[ilat_2x2,ilon_2x2] = mean(tst[ilat+0:1,ilon+0:1])
>  }
>}
>
>tst
>tst_2x2
>
>> tst
>     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
>[1,]    1    5    9   13   17   21   25   29
>[2,]    2    6   10   14   18   22   26   30
>[3,]    3    7   11   15   19   23   27   31
>[4,]    4    8   12   16   20   24   28   32
>
>> tst_2x2
>     [,1] [,2] [,3] [,4]
>[1,]  3.5 11.5 19.5 27.5
>[2,]  5.5 13.5 21.5 29.5
>
>
>I though a cast to 3d-array might do the trick and apply over the new
>dimension, but that does not work, since it casts the data along the
>row.
>> matrix(apply(array(tst,dim=c(nx,ny,8)),3,mean),nrow=nrow(tst)/ny)
>     [,1] [,2] [,3] [,4]
>[1,]  2.5 10.5 18.5 26.5
>[2,]  6.5 14.5 22.5 30.5
>
>
>cheers
>Peter
>
>______________________________________________
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>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>https://stat.ethz.ch/mailman/listinfo/r-help
>PLEASE do read the posting guide
>http://www.R-project.org/posting-guide.html
>and provide commented, minimal, self-contained, reproducible code.