Hi,
Bill was faster than me in suggesting aperm() instead of apply(),
however, his solution is still suboptimal. Try to avoid array(), and
set the dimensions directly if possible.
----
fn1 <- function(x) {
apply(x, 3, t)
}
fn2 <- function(x) {
array(aperm(x, c(2, 1, 3)), c(prod(dim(x)[1:2]), dim(x)[3]))
}
fn3 <- function(x) {
x <- aperm(x, c(2, 1, 3))
dim(x) <- c(prod(dim(x)[1:2]), dim(x)[3])
x
}
# check that the functions return the same
x <- array(1:18, dim=c(3, 2, 3))
stopifnot(identical(fn1(x), fn2(x)))
stopifnot(identical(fn1(x), fn3(x)))
# create two larger arrays, play with the size of the 3rd dimension
x <- array(1:18e4, dim=c(3, 2e1, 3e3))
y <- array(1:18e4, dim=c(3e3, 2e1, 3))
# and the timing:
library(microbenchmark)
microbenchmark(fn1(x), fn2(x), fn3(x), fn1(y), fn2(y), fn3(y), times = 100L)
---
Conclusion:
fn3() is about 3x as fast as fn2(), and fn1() can be extremely
inefficient if dim(x)[3] is large.
HTH,
Denes
On 10/20/2015 08:48 PM, William Dunlap wrote:
Or use aperm() (array index permuation):
> array(aperm(x, c(2,1,3)), c(6,3))
[,1] [,2] [,3]
[1,] 1 7 13
[2,] 4 10 16
[3,] 2 8 14
[4,] 5 11 17
[5,] 3 9 15
[6,] 6 12 18
Bill Dunlap
TIBCO Software
wdunlap tibco.com
On Tue, Oct 20, 2015 at 11:31 AM, John Laing <john.la...@gmail.com> wrote:
x <- array(1:18, dim=c(3, 2, 3))
x
, , 1
[,1] [,2]
[1,] 1 4
[2,] 2 5
[3,] 3 6
, , 2
[,1] [,2]
[1,] 7 10
[2,] 8 11
[3,] 9 12
, , 3
[,1] [,2]
[1,] 13 16
[2,] 14 17
[3,] 15 18
apply(x, 3, t)
[,1] [,2] [,3]
[1,] 1 7 13
[2,] 4 10 16
[3,] 2 8 14
[4,] 5 11 17
[5,] 3 9 15
[6,] 6 12 18
On Tue, Oct 20, 2015 at 12:39 PM, Chunyu Dong <dongchunyu2...@163.com>
wrote:
Hello!
Recently I am trying to transfer a large 3-dimensional array to a matrix.
For example, a array like:
, , 1
[,1] [,2]
[1,] 1 4
[2,] 2 5
[3,] 3 6
, , 2
[,1] [,2]
[1,] 7 10
[2,] 8 11
[3,] 9 12
, , 3
[,1] [,2]
[1,] 13 16
[2,] 14 17
[3,] 15 18
I would like to transfer it to a matrix like:
1 7 13
4 10 16
2 8 14
5 11 17
3 9 15
6 12 18
Could you tell me how to do it in R ? Thank you very much!
Best regards,
Chunyu
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
______________________________________________
R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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