Hello,
Your way is much better than to mess with the dim attribute, like I did.
But,
"If you can create a data.frame or matrix that has the indices"
Actually, it must be a matrix, indices can't be of type list.
A way to avoid loops/apply altogether, and much faster, is the one
creating K3
(K is the result from the op.)
n <- 20
t2 <- system.time({
K2 <- array(0,dim=c(n,n,n,n))
mtx <- data.matrix( expand.grid(x1=1:n,x2=1:n,y1=1:n,y2=1:n) )
K2[mtx] <- apply(mtx, 1, function(x) x["x1"]*x["y2"] -
sin(x['x2']*x['y1']) )
})
t3 <- system.time({
K3 <- array(0,dim=c(n,n,n,n))
mtx <- data.matrix( expand.grid(x1=1:n,x2=1:n,y1=1:n,y2=1:n) )
K3[mtx] <- with(data.frame(mtx), x1*y2 - sin(x2*y1))
})
It's also woth noting that both use much more memory than the loop
version because the index matrix can be large.
Rui Barradas
Em 15-05-2012 11:00, r-help-requ...@r-project.org escreveu:
Date: Mon, 14 May 2012 13:22:11 -0400 From: David Winsemius
<dwinsem...@comcast.net> To: math_daddy <math_da...@hotmail.com> Cc:
r-help@r-project.org Subject: Re: [R] How to apply a function to a
multidimensional array based on its indices Message-ID:
<155f48f0-04c2-4033-a7be-fc5109224...@comcast.net> Content-Type:
text/plain; charset=US-ASCII; format=flowed; delsp=yes On May 14,
2012, at 10:09 AM, math_daddy wrote:
> Hello. I have a 4 dimensional array and I want to fill in the slots
> with
> values which are a function of the inputs. Through searching the
> forums here
> I found that the function "outer" is helpful for 2x2 matrices but
> cannot be
> applied to general multidimensional arrays. Is there anything which
> can
> achieve, more efficiently than the following code, the job I want?
>
> K<- array(0,dim=c(2,2,2,2)) #dimensions will be much larger
> for(x1 in 1:2)
> {
> for(y1 in 1:2)
> {
> for(x2 in 1:2)
> {
> for(y2 in 1:2)
> {
> K[x1,y1,x2,y2]<- x1*y2 - sin(x2*y1) #this is just a dummy
> function.
> }
> }
> }
> }
>
If you can create a data.frame or matrix that has the indices
x1,x2,y1,y2 and the values you can use the: K[cbind(index-vectors)]
<- values construction:
mtx<- data.matrix( expand.grid(x1=1:2,x2=1:2,y1=1:2,y2=1:2) )
K[mtx]<- apply(mtx, 1, function(x) x["x1"]*x["y2"] -
sin(x['x2']*x['y1']) )
#----------------
> K
, , 1, 1
[,1] [,2]
[1,] 0.158529 0.09070257
[2,] 1.158529 1.09070257
, , 2, 1
[,1] [,2]
[1,] 0.09070257 1.756802
[2,] 1.09070257 2.756802
, , 1, 2
[,1] [,2]
[1,] 1.158529 1.090703
[2,] 3.158529 3.090703
, , 2, 2
[,1] [,2]
[1,] 1.090703 2.756802
[2,] 3.090703 4.756802
> Thank you in advance for any help.
>
>
> --
> View this message in
context:http://r.789695.n4.nabble.com/How-to-apply-a-function-to-a-multidimensional-array-based-on-its-indices-tp4629940.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guidehttp://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
David Winsemius, MD
West Hartford, CT
______________________________________________
R-help@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.
