R help - Mar 2009 - How to apply a function to slices of multidimensional arrays, and not just iterating through single elements

Hello,

If I want to apply some f(x) to such chunks of the the array
dim(A)==c(d1,d2,d3,..,dk,...,dn) which are defined by A[...,ik,...]
(ik belongs to {1,..,dk}), for now I use iteration via 'for (i in
dim(A)[k]) f(A[...,k,...])' . Is there any more elegant approach, e.g
like in 'apply' function which you can use on margin of the array.
Just in my case I want the entire slice defined by margin to be an
argument to my f(x), not just element-by-element.

If the former is too confusing:
A - array
dim(A)=c(3,4,5)
f(x) - function; x is argument dim(x)=c(3,5)
A has 4 slices of dim c(3,5)
I want my result to be a vector c( f(A[,1,]), f(A[,2,]), f(A[,3,]), f(A[,4,]) )
Until now I was doing 'for (i in 1:4) res[i]<-f(A[,i,])' . Is there a
more efficient way of doing this, maybe some predefined function?

Cheers,

TK

well, you can still use apply(), e.g.,

A <- array(rnorm(3*4*5), c(3, 4, 5))
f <- sum

out <- numeric(4)
for (i in 1:4)
     out[i] <- f(A[, i, ])
out
apply(A, 2, f)


I hope it helps.

Best,
Dimitris


Todor Kondic wrote:
> Hello,
> 
> If I want to apply some f(x) to such chunks of the the array
> dim(A)==c(d1,d2,d3,..,dk,...,dn) which are defined by A[...,ik,...]
> (ik belongs to {1,..,dk}), for now I use iteration via 'for (i in
> dim(A)[k]) f(A[...,k,...])' . Is there any more elegant approach, e.g
> like in 'apply' function which you can use on margin of the array.
> Just in my case I want the entire slice defined by margin to be an
> argument to my f(x), not just element-by-element.
> 
> If the former is too confusing:
> A - array
> dim(A)=c(3,4,5)
> f(x) - function; x is argument dim(x)=c(3,5)
> A has 4 slices of dim c(3,5)
> I want my result to be a vector c( f(A[,1,]), f(A[,2,]), f(A[,3,]),
f(A[,4,]) )
> Until now I was doing 'for (i in 1:4) res[i]<-f(A[,i,])' . Is
there a
> more efficient way of doing this, maybe some predefined function?
> 
> Cheers,
> 
> TK
> 
> ______________________________________________
> R-help at r-project.org mailing list
> 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.
> 
-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014

If I well understand, maybe you have still apply at your disposal:

arrayA <- 1:60
dim(arrayA) <- c(3,4,5)
apply(arrayA, 2, sum)

You have the same result of:
res<-numeric(4);for (i in 1:4) res[i]<-sum(arrayA[,i,])

Ciao,
domenico

PS:
have a look at plyr package for more "slicing" and
"applying" functions

Todor Kondic wrote:
> Hello,
>
> If I want to apply some f(x) to such chunks of the the array
> dim(A)==c(d1,d2,d3,..,dk,...,dn) which are defined by A[...,ik,...]
> (ik belongs to {1,..,dk}), for now I use iteration via 'for (i in
> dim(A)[k]) f(A[...,k,...])' . Is there any more elegant approach, e.g
> like in 'apply' function which you can use on margin of the array.
> Just in my case I want the entire slice defined by margin to be an
> argument to my f(x), not just element-by-element.
>
> If the former is too confusing:
> A - array
> dim(A)=c(3,4,5)
> f(x) - function; x is argument dim(x)=c(3,5)
> A has 4 slices of dim c(3,5)
> I want my result to be a vector c( f(A[,1,]), f(A[,2,]), f(A[,3,]),
f(A[,4,]) )
> Until now I was doing 'for (i in 1:4) res[i]<-f(A[,i,])' . Is
there a
> more efficient way of doing this, maybe some predefined function?
>
> Cheers,
>
> TK
>
> ______________________________________________
> R-help at r-project.org mailing list
> 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.
>
>