[R] Aggregate matrix in a 2 by 2 manor

[David Winsemius]

> On Jul 27, 2016, at 12:02 PM, Jeff Newmiller <jdnewmil at dcn.davis.ca.us> wrote:
> 
> 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 )

Here's an apparently loopless strategy, although I suspect the code for interaction (and maybe tapply as well?) uses a loop.


tst_2X2 <- matrix(NA, ,ncol=4,nrow=2)

 tst_2x2[] <- tapply( tst, interaction( (row(tst)+1) %/% 2, (col(tst)+1) %/% 2 ), mean)

 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.


> 
> -- 
> 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.
> 
> ______________________________________________
> 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.

David Winsemius
Alameda, CA, USA